This study examines the effects on occupational injury claims of a recently-implemented post-accident drug testing (PADT) program in a large Fortune 500 retail chain with homogeneous facilities in a large number of U.S. states. We find that accident-related claims have fallen significantly in affected districts during the first fifteen months of the program. The observed drop in claims was driven mostly by the behavior of male, higher-tenure, and full-time employees. Moreover, conditional on being tested, full-time, higher tenure, and female workers were the least likely to test positive for illegal drugs. Overall, our results lend considerable credence to the view that PADT programs can reduce workers' compensation claims even in workplaces that already utilize other forms of employee drug testing, particularly among workers who value their jobs the most highly. We also attempt to distinguish among two possible explanations for the observed declines: employees' tendency to forgo using drugs and/or take greater care on the job; and a fall in employees' willingness to report accidents. Our results, although certainly not definitive, suggest that at least some portion of the observed decline may be due to underreporting. We conclude that although PADT is a promising method of improving occupational safety, it raises special policy concerns insofar as it may encourage some employees to hide their injuries.