A cross-over study carried out over a 24 week period during 1975-76 was designed to test the relative merits of Extracorporeal versus Travenol coils for thrice weekly short periods of haemodialysis for patients with end-stage renal failure. Predictability of and capacity for ultrafiltration were excellent and equally good for both products. Leak rates in Travenol coils were 9.3% as compared to 4.3% for Extracorporeal coils. Dialysances of BUN, creatinine, and phosphate were significantly higher with Travenol coils than with Extracorporeal coils, but this increase was not as great as might be expected from the greater surface area of the Travenol coil. Despite the greater functional efficiency of the Travenol coils we could detect no difference in the degree of biochemical control of the patients as judged by standard pre and post dialysis blood chemistries. Residual blood volumes averaged 9 mls for the Travenol coils as compared to 1.6 mls for Extracorporeal coils.