To determine if dialysis modality may be an independent factor in the level of cognitive function in chronic dialysis patients, cognitive function was studied in 17 pairs of continuous ambulatory peritoneal dialysis (CAPD) and center hemodialysis (CHD) subjects matched for sex, age, diabetic status, and interval since dialysis onset. Data on current metabolic, medical, psychological, and vocational function status were obtained. Neuropsychological (NP) measures included the Number Cancellation Protocol (NCP), Trailmaking test forms A and B (TMT A, TMT B), Symbol Digit Modalities (SDM), and the Rey Auditory Verbal Learning Test (RAVLT). The CAPD subject group had consistently more efficient cognitive function than the CHD subject group. Regardless of modality, the groups of subjects under age 51 and those who were vocationally active had significantly better NP performance. No cognitive function differences were found in groups categorized by sex or duration of dialysis. Creatinine levels were more highly correlated with NP scores than were BUN levels, with higher creatinine levels associated with better cognitive function. Serum calcium, CO2, total protein, albumin, and SGOT levels also were correlated with NP scores. CAPD may be more effective than HD in reversing uremic encephalopathy by mechanisms mostly unrelated to serum creatinine and BUN levels. Longitudinal studies will be needed to determine if dialysis modality is an independent factor in the degree of reversal of uremic encephalopathy.