Detailed analysis of work exposure and worker attributes failed to explain changes in injury claims in three plants, each in a separate state, during 1973-1980. When relative exposure to hazard and worker attributes are controlled, changes in injury rates are largely explained by increases in Workers' Compensation greater than inflation and by inspections made by the Occupational Safety and Health Administration (OSHA). Rises in Workers' Compensation greatly increased the claims for injuries, while OSHA citations substantially decreased objectively verifiable injuries in the year following an inspection. Data on days lost due to injury in 167 industrial groupings in 20 states also indicate significant reductions of workdays lost in correlation with OSHA inspections, when increases due to Workers' Compensation are controlled. The results thus appear to be generalizable. Previous studies of OSHA failed to adequately control for Workers' Compensation effects, and thus underestimated the effects of OSHA.