The role of employment status (active or inactive) in mortality studies was investigated in ten large occupational cohorts conducted by NIOSH. The ten studies used involved workers in the following industries: pesticide manufacturing, paper and pulp manufacturing, mining, rubber manufacturing, chemical manufacturing, shipbuilding, automobile manufacturing, and munitions. No occupational risks were observed in these studies. The data sets involved information on 89.376 workers over 1,984,505 person years. During inactive person years the all causes standard mortality ratio (SMR) was 1.12. A marked increase in mortality was noted during the first year following employment when the data was stratified for inactive person years by time since last employment. Using the data for active person years the all causes SMR was 0.40 and was fairly constant across the various age categories. A strong negative trend in SMRs was noted when the active and inactive person years were combined for the duration of employment for all causes and for heart disease. When person years were stratified by employment status these trends were not apparent. The authors conclude that the effects of employment status should be evaluated when comparing SMRs between multiple cohorts or when interpreting trends in rate ratios within cohorts.