A study of the long time effects of changing from an 8 to 12 hour workshift was conducted. This was a followup to an earlier study of 20 control room operators (18 males), 26 to 41 years old, at a continuous processing facility after they had switched from an 8 hour shift to a 3 to 4 day rotating 12 hour shift. In the earlier study which was conducted after they had been on the 12 hour shift schedule for 7 months decrements in performance and alertness and a mild to moderate sleep deficit had been found. The present study was conducted after the workers had been on the 12 hour schedule for 3.5 years. The workers completed a computerized fatigue test battery that measured cognitive ability, auditory reaction times, and hand steadiness at various times during the work week. They completed a questionnaire to assess daily sleep, psychological stress, gastrointestinal symptoms, and personal schedule adjustments attributable to shiftwork. Significant decrements were seen on all tests in the fatigue test battery compared to those measured when the subjects were on the 8 hour shift. The decrements did not vary significantly across the work week. The subjects reported an average 1 to 3 hour sleep deficit. Self reported stress was significantly less during the early part of the work week but increased as the week progressed. The author concludes that decrements in performance and alertness and decreases in sleep time still persist after switching to the 12 hour shift schedule. Because of the reduced sleep time and resulting fatigue it causes, workers on 12 hour shifts should attempt to obtain more sleep during the work week.