Using data generated in the course of an evaluation program for hemodialyzers and associated devices, which was supported by the Medical Devices Agency of the Department of Health, we have compared the performance characteristics of 23 hemodialyzers evaluated 20 years ago with those of 18 hemodialyzers evaluated during 1993-1994. Surface areas of the two groups ranged from 0.6-2.5 m2 (mean 1.13 m2) and 0.6-1.8 m2 (mean 1.18 m2), respectively. The device performance has been compared both in vivo (small molecular clearance) and in vitro (middle molecular clearance and ultrafiltration coefficient) at blood and dialysate flows of 200 and 500 ml/min, respectively. Our comparison shows that over the past 2 decades there has been a marked improvement in functional performance. For clearance of small molecules, current dialyzers using cellulose-based membranes offer a 47% improvement for urea clearance, within the surface area range < 1.2 m2, while for dialyzers whose surface area is > 1.2 m2 the improvement is less marked (12%). The improvements noted in the clearance of creatinine were similar. Associated with the improved clearance of small molecules was an improved middle molecular clearance (74% for surface areas < 1.2 m2 and 63% for surface areas > 1.2 m2) and an increased ultrafiltration coefficient, higher by 154 and 133%, respectively, for the two surface area groups. Due to the smaller number of devices in the modified cellulose and synthetic membrane categories, it has not been possible to make such a detailed comparison. However, the experimental data demonstrate that devices currently produced incorporating modified cellulose and synthetic membranes overlap in performance and that in those devices that were in clinical use 20 years ago using synthetic membranes, current equivalents have also improved. These improvements are reflected in the concomitant reduction of treatment times from 30 h/week in 1970 to 12 h or less currently.