Clearance studies of urea, creatinine and inorganic phosphate have been carried out using polycarbonate dialysers on 20 occasions in two patients undergoing routine maintenance haemodialysis. Clearances were calculated using plasma concentrations and both measured blood flow and calculated plasma flow. Urea and creatinine clearances were comparable to those obtained with cellulose-based dialysers, but phosphate clearances were relatively increased, averaging over 70% of urea clearances. All clearances were reduced as TMP values increased, a feature at variance with the usual situation with cellulose membranes. Polycarbonate dialysers have the advantage of higher relative phosphate clearances in addition to the other advantages claimed for them.