Chronic renal failure in young children is associated with impaired cognitive development, but recent studies present a more optimistic perspective. An important question is whether the earlier initiation of renal replacement therapy (RRT) might prevent the reported developmental retardation. The cognitive development of 31 patients (age < 5 years with a serum creatinine clearance of < 20% of normal) undergoing different treatment modalities was monitored by repeated measurements during a prospective 3-year study. Fifteen patients received conservative treatment and 16 patients were on dialysis treatment at the start of the project. We were able to evaluate the effect of the onset of RRT on 12 patients who were transferred from conservative treatment to dialysis. At the beginning of the study, the cognitive development of the total group was significantly delayed (mean developmental index = 78.5, SD = 19.5) compared with a normal population. Patients undergoing conservative treatment scored significantly higher (P < 0.01) than those on dialysis. The effect of starting dialysis treatment appeared to be positive, but only a significant short-term improvement was observed. Follow-up evaluation of 7 patients on conservative treatment and of 9 dialysis patients over a 2-year period did not show any significant change in a positive or negative direction. The present study revealed that pre-school dialysis patients are at risk with respect to their cognitive development. This is particularly true for the group with concomitant disorders. Less severe disease in the group on conservative treatment may be assumed to be a positive contributing factor to the more normal performance of these patients.(ABSTRACT TRUNCATED AT 250 WORDS)