Aluminum (Al) concentration in serum, urine, and dialysate was estimated in 21 patients undergoing continuous ambulatory peritoneal dialysis (CAPD). In 12 of the patients bone Al concentration was measured as well. Mean serum Al level was 32.4 +/- 21.0 micrograms/l. The Al concentrations in the dialysate and urine were 9.1 +/- 4.1 micrograms/l and 52.5 +/- 47.3 micrograms/l, respectively. Bone Al concentration was 21.0 +/- 14.9 ppm and correlated significantly with concentrations of Al in serum (p less than 0.01) and dialysate (p less than 0.01). A mass transfer (MT) from the patients to the dialysate was observed in all patients (-44.0 +/- 28.8 micrograms/24 h). There was a highly significant correlation between peritoneal Al MT and serum Al (p less than 0.001), actual Al consumption (p less than 0.05) and bone Al concentration (p less than 0.005) supporting the existence of an overflow phenomenon. Despite very low Al levels in the dialysate, patients are at risk of elevated Al levels in the serum, dialysate, urine and bone because of consumption of Al-containing phosphate binders.