We have previously derived an index, based on retrospective data, for mortality in patients with end-stage renal disease (ESRD) treated by dialysis and transplantation. We used this index to calculate probability of death and rates of hospitalization, two measures of severity of illness, for 436 patients enrolled in our ESRD program after the original index was derived. Applied when ESRD treatment was initiated, it predicted future mortality and hospitalization rates. We then analyzed clinical characteristics, including variables in the predictive model, in all 718 patients enrolled in 3-year cohorts from 1976 to 1989. Over time, there was trend toward enrolling patients with a higher likelihood of dying, ie, more severely ill. The severity index facilitated description of the patients and their changing characteristics over time, and proved useful in comparing the degree of illness in different population groups.