Twelve years' experience with a Phase I drug testing program in normal prison volunteers is reported. Involved in 805 protocol studies were 29,162 participants over 614,534 subject days. During this period there were 64 significant medical events of which 58 were adverse drug reactions and 6 were complications. One subject has residual hip changes due to an infectious complication, another on placebo died from cerebrovascular hemorrhage while asleep. There was complete recovery from all adverse drug reactions and the other 4 complications encountered. Thus a clinically significant medical event occurred once every 9,602 days subject exposure or about once every 26.3 years of individual subject participation.