PurposeThe role of labor unions in relation to corporate social responsibility (CSR) remains both ambiguous and crucial for union members and business leaders. Given the complex relationship between labor unions and corporations, this study aims to address whether labor unions keep corporations honest (by monitoring CSR activities) or potentially render CSR initiatives less necessary.Design/methodology/approachUsing data from the MSCI Kinder, Lydenberg, Domini Database for firms in the Russell 1000 Index, this study examines the link between labor unions and CSR in U.S. companies over a six year period. Generalized least squares models were used to test the hypotheses for 3,937 firm-year observations.FindingsThe findings show that unionized companies generally pay less attention to CSR compared to nonunionized ones. The presence of labor unions and positive union-management relations both show a significant negative impact on CSR ratings, where positive union-management relations negatively affect CSR ratings more than just the presence of labor unions. Further, when considering the environmental, social and governance aspects of CSR separately, the results are more complex, suggesting that the relationship between labor unions and CSR varies depending on specific ESG dimensions.Originality/valueCSR, a well-researched area, rarely addresses the companies' relationships with labor unions. Studies in South Korea and the UK have touched on the impact of labor unions on CSR, but in the USA it remains unexplored. This study extends this line of work by examining U.S. companies.