PURPOSE: To assess the incidence of surgically induced miosis during phacoemulsification in diabetic patients. METHODS: A total of 76 patients with diabetes mellitus were compared to 76 age- and race-matched controls. A combination of cyclopentolate 1%, phenylephrine 2.5% and diclofenac sodium 0.1% was applied topically 60, 45 and 30 min before surgery. Adrenaline mixed with buffered saline solution was used for irrigation during surgery. The procedure included phacoemulsification and implantation into the bag of a foldable acrylic implant. Measurements of the horizontal pupillary diameter were taken at three stages: before corneal incision, after phacoemulsification, and at the end of surgery. The duration of phacoemulsification was also recorded. RESULTS: Surgically induced miosis or dilation of the pupil was defined as constriction or dilation noted at any interval during surgery. The pairs of diabetic-control were grouped into three groups: those in which constriction was noted, those in which dilation was noted, and those in which there was no change in pupil size during the procedure. Surgically induced miosis was noted more often in the diabetics (McNemar's test, chi(2), P=0.016). The mean pupil size at the beginning of surgery was 7.38 (+/-0.95) mm in the diabetics as compared to 7.65 (+/-0.89) mm in the control group. No statistically significant difference was noted between the two groups (paired t-test, P=0.07). The mean (+/-SD) duration of phacoemulsification in the diabetic group was 2.31 (+/-1) min as compared to 2.05 (+/-0.82) min in the control group. No statistically significant difference was found between the two groups (paired t-test, P=0.08). CONCLUSION: Surgically induced miosis occurred more often in the diabetics. Therefore, it is advisable that phacoemulsification in this group of patients is undertaken by an experienced surgeon.