The clearance of organic ions by the tubules may contribute to the removal of uremic waste products in dialysis patients. The renal excretion of an exogenous anion p-aminohippurate (PAH) was investigated in 10 peritoneal dialysis patients and 10 hemodialysis patients during one clearance period and compared with the clearance of creatinine (Ccr) and inulin (CIn). The clearance period was 24 hours in the peritoneal dialysis patients and one interdialytic interval of 3 days divided in 4 parts [CPA-D] in hemodialysis patients. In peritoneal dialysis patients the renal clearance of total PAH (median 14.3 ml/min, range 3.8-33.0) exceeded the CIN (median 3.2 ml/min, range 1.6-11.2, p < 0.005) and Ccr (median 4.0 ml/min, range 1.7-15.0, p < 0.005). A positive correlation was found between the tubular clearances of creatinine (cationic pathway) and of total PAH (anionic pathway, r: 0.72, p <0.02). In hemodialysis patients the clearance of total PAH (CPA: median 2.0, range 0.8-9.6; CPD: median 3.8, range 1.7-15.4) also exceeded the clearance of inulin (CPA: median 1.5, range 0.2-3.4; CPD: median 2.7, range 0.9-4.4) in the beginning and the end of the interdialytic interval (p < 0.005). The CIN and the clearance of total PAH increased during the interdialytic interval, but the Ccr (CPA: median 2.2, range 0.4-8.9, CPD: median 2.9, range 1.2-4.6) remained stable. Thus, the change in tubular clearance of creatinine and PAH was opposite during the interdialytic interval: it increased for total PAH and decreased for creatinine. The CTPAH/CIN ratio in hemodialysis patients was lower than in peritoneal dialysis patients. In CPA it was median 1.6 (range 1.1-5.6, p < 0.05) and in CPD it was median 1.7 (range 1.1-5.0, p < 0.02) and in the peritoneal dialysis patients it was median 3.6 (range 1.5-9.1). We conclude that tubular clearances contribute to the residual renal function in dialysis patients, but the tubular handling of anions and cations in relation to the residual GFR is different between peritoneal and hemodialysis patients. A difference in clearance of organic acids caused by the dialysis techniques may be an explanation for the differences in clinical outcome between the two dialysis modalities.