Abstract Purpose: To determine whether systemic diclofenac sodium affects the maintenance of mydriasis during phacoemulsification and to compare pupil diameter regression rates with those in a control group. Setting: An ophthalmic unit of a National Health Service Trust Hospital in the United Kingdom. Method: A prospective, randomized control study of 100 patients presenting for cataract surgery was undertaken. Both groups were administered an identical mydriatic regime commencing 1 hour before surgery. In addition, the diclofenac group (n = 48) received 50 mg of oral diclofenac sodium 1 hour before surgery. Both the control (n = 52) and diclofenac groups had phacoemulsification by one experienced surgeon using a standard technique. The surgeon was masked as to which group the patient belonged. The irrigating fluid in all cases contained adrenaline. Pupil diameter was measured at five stages during surgery and recorded along with the times relative to the start of the procedure. Results: Mean pupil diameter in the diclofenac group was slightly larger than in the control group, but the difference was not significant. Although mean pupil diameter at the start of surgery was 0.4 mm greater in the diclofenac group, the slopes of regression in pupil diameters were virtually identical between the first three stages of surgery, after which a slight reversal occurred in both groups. The slope of regression of mydriasis was small in both groups. Conclusion: Systemic diclofenac sodium 50 mg given orally 1 hour before surgery did not significantly inhibit miosis when compared with a control group. Regression rates of pupil mydriasis were small in both groups.