Objective: To examine whether the inverse care law operates in a screening program for diabetic retinopathy (DR) based on fee for service in Hong Kong. Design: Randomized controlled trial. Participants: All those with type 1 or 2 diabetes from 2 clinics were recruited. Intervention: Diabetic retinopathy screening with a small copayment versus free access in a publicly funded family medicine service. Main Outcome Measures: Uptake of screening and severity of DR detected. Association between these outcome variables and independent variables were determined using multivariate logistic regression models and reported as odds ratios (ORs). Results: After randomization, 1387 subjects in the free group and 1379 subjects in the pay group were eligible for screening, and 94.9% (1316/1387) and 92.6% (1277/1379), respectively, agreed to participate in the study. The offer of screening was accepted by 94.8% (1247/1316) in the free group and 91.2% (1164/1277) in the pay group, and the final uptake ratios were 88.5% (1165/1316) and 82.4% (1052/1277), respectively (Pearson chi = 19.74, P<0.001). Being in the pay group was associated with a lower uptake of screening than being in the free group (OR, 0.59; confidence interval [CI], 0.47-0.74) and a lower detection rate of DR (OR, 0.73; CI, 0.60-0.90) after adjustment for potential confounding factors. Subjects with higher socioeconomic status were more likely to attend screening and had a lower prevalence of DR detected. Conclusions: The inverse care law seems to operate in a preventive intervention when a relatively small copayment is applied. There is a case for making effective preventive services free of charge. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article.