Decrease in the average duration of hemodialysis treatment time is a continuing phenomenon. We investigated the relationship of 3-year mortality to duration of dialysis in a 1984-1985 national random sample of 600 hemodialysis patients from 36 dialysis units. Mortality was negatively associated with duration of dialysis treatments, as shown by the Cox model, adjusted for other patient and dialysis unit covariates. With adjustment for other covariates, patients receiving an average dialysis treatment duration of less than 3.5 hours had relative mortality risks of 1.17 to 2.18 compared with those with treatments longer than 3.5 hours (mortality risk of 1.0). Reverse causation (the possibility that more seriously ill patients received dialysis for a shorter time) appears unlikely. We conclude that duration of the dialysis procedure is an important element in determining patient mortality as one of the factors determining the adequacy of dialysis.