Hemodiafiltration, a process in which whole blood is first diluted with a physiologic electrolyte solution and then ultrafiltered across a membrane to convectively remove solutes and excess water, has been applied clinically for the first time. Six-hour hemodiafiltration with a 1.6 m.2 hollow-fiber ultrafilter was applied intermittently to an anephric patient as an alternative to 6-hour hemodialysis using a 1.45 m.2 coli. A quantitative basis for evaluating clinical hemodiafiltration kinetics was developed, and the results were compared with data from prototype devices. With blood, diluting fluid, and ultrafiltrate flow rates of 200 ml. per minute, removal rates of urea and creatinine by both hemodiafiltration and dialysis were comparable, but for solutes of larger molecular weight (uric acid, phosphate, and inulin) removal rate was significantly greater for hemodiafiltration. The observed ultrafiltrate flux was similar to values predicted from in vitro studies. With the present membrane formulation the measured sieving coefficients for inulin, creatinine, and urea were not significantly different from one, and whole blood clearances for these solutes were 117, 108, and 101 ml. per minute, respectively. This solute clearance pattern is very similar to the human kidney and in sharp contrast to standard coil hemodialysis.