Recirculation studies were performed in 103 patients treated with high-efficiency dialysis over a 14 month period. Fistulograms were performed on 22 out of 25 patients with greater than 0.15 fractional recirculation at a 400 ml/minute blood pump setting. Clinically significant abnormalities were found in 82% (N = 18) and treated in 17. Two patients had second episodes of elevated recirculations and were treated again within the period of follow-up. Treatment with angioplasty (N = 11) or surgical revision (N = 8) resulted in a fall in recirculation from 0.33 ± 0.04 to 0.12 ± 0.02 (P = 0.001). The fractional reduction of urea clearance due to recirculation fell from 0.20 ± 0.03 to 0.08 ± 0.02 (P = 0.001) and the effective urea clearance of the dialysis treatment rose by 16% from 193 ± 7 ml/min to 224 ± 6 ml/min (P = 0.001). Pre-dialysis BUN fell from 72 ± 4 mg/100 ml to 62 ± 3 mg/100 ml (P = 0.012). There was no correlation between venous pressure (VP) at 400 ml/min blood pump setting and recirculation (R2 = 0.04), although VP changed significantly comparing values before and after fistula repair (211 ± 10 vs. 186 ± 7 mmHg, P = 0.012). Venous pressures in 20 of the patients in our dialysis unit with recirculations of <0.10 were 201 ± 6 mmHg (P = NS compared to patients with recirculation ≥0.15 at 400 ml/min blood flow). We conclude that: a) significant reductions of fistula flow rates as determined by increased recirculation occurs in the absence of other abnormalities in fistula function such as increased VP or thrombosis; and b) correction of fistula abnormalities detected by increased recirculation improves the quality of the dialysis procedure by increasing effective clearance of solutes.