BACKGROUND Breakdown of the blood-aqueous barrier (BAB) after panretinal photocoagulation (PRP) was measured with a laser flare photometer over a study period of 8 weeks. METHODS Twenty-five eyes of 25 patients who had no previous photocoagulation and required such treatment for proliferative diabetic retinopathy (PDR) were included in the trial. They received 2000 burns (0.1-second exposure, 200 mu m spot) via a panfunduscope and 500 burns (0.1-second exposure, 500-mu m spot) with a Goldmann lens. Power levels were adjusted to produce a mild blanching of the retina. Only an argon green laser (514 nm) was used. Laser photometry was performed on both eyes at 3, 24, 48, 72, 96, and 168 hours and 8 weeks after laser treatment. RESULTS Including all of the eyes treated, there was a significant increase in flare value of 3, 24, and 48 hours compared with baseline (Student's t test) but not at 72, 96, and 168 hours or at 8 weeks. Peak values occurred at 24 hours. When blue and brown irides were analyzed separately, there was a significant increase in flare for blue irides compared with baseline levels at 3 and 24 hours, whereas for brown irides the increased flare was sustained at 3, 24, 48, 72, and 96 hours (Student's paired t test). In addition, when the increase in flare value from baseline was compared between blue and brown irides (pooled Student's test), there was a sustained increase at 24, 48, 72, and 96 hours for brown irides compared with blue. Clinically significant uveitis, posterior synechiae, or peripheral anterior synechiae did not develop in any of the patients. CONCLUSIONS Breakdown of the BAB may occur after PRP, particularly in more heavily pigmented irides. The time course of this suggests that the phenomenon is related directly to laser effects in the anterior segment, although other factors may contribute.