Two types of intraocular lenses (IOLs) were used in a controlled, randomized, double-masked clinical trial. The study lens was a one-piece, poly(methyl methacrylate) (PMMA) lens with heparin grafted on the entire surface. The control lens was identical in all respects but lacked the heparin surface. The IOLs were implanted in the capsular bag after extracapsular cataract extraction in 266 patients. No unexpected reactions or severe complications occurred. The postoperative visual acuity of all cases at the three-month visit was 20/40 or better in 91.6% of the heparin group and in 86.2% of the control group. When excluding cases with preoperative pathology, 97% of the heparin group and 95% of the control group had a visual acuity of 20/40 or better. A mild iritis was observed in one case of the heparin group and in four cases of the control group at three months. At three months there was a statistically significant difference between the heparin surface modified and control IOLs for both the number of patients with cellular precipitates and the number of precipitates per IOL. From the present data we conclude that during the first three postoperative months heparin surface modified lenses are more biocompatible than PMMA lenses.