PURPOSE: To evaluate histopathologic signs of conjunctival inflammation in patients with conjunctival hyperemia induced by bimatoprost treatment. DESIGN: Prospective interventional study. METHODS: The study included 15 eyes of 15 patients scheduled for cataract surgery. Patients in the treatment group (n = 9) exhibited trace to moderate conjunctival hyperemia when treated with bimatoprost 0.03% every day for 15 to 30 days before surgery. The control group (n = 6) included untreated patients with no ocular disease other than cataract. Conjunctival biopsies were obtained for histologic evaluation with light microscopy. RESULTS: Vascular congestion was observed in biopsies from 7 patients (78%) in the bimatoprost group and 5 patients (83%) in the control group. Signs of inflammation were found in biopsies from 2 patients (22%) in the bimatoprost group and 2 patients (33%) in the control group. CONCLUSIONS: Histopathologic signs of inflammation were no more frequent in conjunctival specimens from bimatoprost-treated patients with trace to moderate hyperemia than in those from untreated control subjects.