In recent years, concern about the economic polarization of large firms and small and medium-sized firms has drawn attention to the supply chain. Despite the economic and social importance of win-win cooperation between large firms and small and medium enterprises (SMEs), due to a lack of data there have been few analyses of the relationship between suppliers and large firms. This study investigates the accounting performance of the suppliers of large firms, and compares it with the accounting performance of the suppliers of other firms. Large firm buyers are defined as industry leading companies (the top two companies in an industry), and suppliers are the companies that sell to those firms. For a company to be defined as a supplier of a large buyer, that company's sales to a large buyer should be more than 10 percent of its total sales. Sales data for the suppliers are taken from KED (Korea Enterprise Data), which provides transaction data from individual firms. Non-suppliers, those who do not sell to the large firms, are matched with the suppliers by asset size and 3-digit industry code. A total of 6,238 firm-year records for suppliers that sold to 104 industry leading companies between 2005 and 2012 are used, and 6,635 firm-year records for non-suppliers are matched with those of the suppliers. Conventionally, the relationship between large companies and their suppliers is explained in terms of relative bargaining power. If a large firm shifts costs to suppliers to make profits or to cover losses, the relationship between the suppliers' margins and those of the large buyers is expected to be negative. However, we find that suppliers' margins have a positive relationship with those of large buyers. Next, we compare the gross margins of suppliers with those of non-suppliers, and find that the gross margins of suppliers are significantly lower than those of non-suppliers. This result is consistent with findings previously reported in the Korean literature, which suggest that large buyers' requests for suppliers to lower their product prices reduces the suppliers' gross margins. Suppliers' and non-suppliers' operating margins, which reflect operating expenses, are not significantly different, but the profit margins of suppliers are significantly higher than those of non-suppliers. Analysis of the margins shows that suppliers' operating and other expenses are lower than those of non- suppliers. A buyer's reputation might reduce its supplier's marketing expenses, which could offset the loss in selling price. Also efficient asset utilization induces cost reductions and can be measured by turnover ratios, we compare turnover ratios for suppliers and non-suppliers. Fewer days in inventory and receivables shortens the cash conversion cycle and improves the management of working capital, which is especially important to SMEs. We find that the asset turnover ratios, inventory turnover ratios and accounts receivables turnover of suppliers are significantly higher than those of non-suppliers. More specifically, the number of days in inventory and in receivables are lower for suppliers than for non-suppliers. Fewer days in inventory lowers the cost of inventory and fewer days in receivables lowers the cost of capital. There is no significant difference in payables turnover between suppliers and non-suppliers. Turnover ratio analysis shows that suppliers make more efficient use of assets than non-suppliers. According to the DuPont Identity, asset turnover increases return on assets (ROA) and return on equity (ROE) even though profit margins are not higher. In our analysis, we find that suppliers' profit margins are higher than those of non-suppliers. Therefore, the better use of assets by suppliers enhances ROA and ROE compared with non-suppliers. Lower gross profit margins and enhanced asset utilization are more common among suppliers that are affiliated with the top four business groups (Samsung, LG, SK and Hyundai Motors). Samples are divided into two groups: suppliers of the top four business groups and their matched non- suppliers, and suppliers of other large firms and their matched non-suppliers. A small profit and quick returns strategy is apparent for the suppliers of the top business groups. This strategy means that the top business groups have more bargaining power than other large buyer firms, but can also help their suppliers to achieve higher sales with the associated benefit of a higher ROA and ROE. In this study, the performance and supplier dummies may lead to an endogeneity problem. However, after controlling for endogeneity using two-stage least squares (2SLS) regression, the results are consistent. To test robustness, the non-supplier sample is restructured according to a propensity scoring match. The main results are also consistent with the analysis of the restructured sample. This study shows that public perceptions of the relationship between large firms and small to medium-sized firms are partially correct. The low gross margins of suppliers can be interpreted as evidence of the bargaining power of large firms. However, suppliers obtain a higher ROA and ROE than non-suppliers due to efficient asset utilization. Given these results, we hope that both the suppliers and the large firms will appreciate each other's role and strengthen their win–win relationships.