OBJECTIVES: The Inverse Care Law, whereby health care services are less accessible to those in greater need, has been found to operate in a variety of health systems.  Tudor Hart asserted that market forces would exaggerate the maldistribution of resources. The objective of this study is to examine whether the inverse care law operates in a screening program for diabetic retinopathy (DR) in Hong Kong (HK) with a mixed medical economy like other Asian countries. METHODS: A randomized controlled trial has been conducted.  All those with Type 1 or Type 2 diabetes from two public primary care clinics in Hong Kong were recruited and then were randomly allocated into one group paying a fee of HK$60 (US$8) for screening  and the other group with free access. The outcome measures are uptake of screening and extent of DR detected. RESULTS: After randomization, 1387 in free and 1379 in pay groups were eligible for screening. 94.9%(1316/1387) and 92.6%(1277/1379)respectively agreed to participate in the study. The final screening uptake rate were 88.5% (1165/1316)and 82.4% (1052/1277)respectively (P<0.001). Being in the pay group was associated with lower uptake of screening than in the free group (OR=0.59, 0.47-0.74) and lower prevalence of DR detected (20.3% VS 25.9%, P=0.004).  Subjects with higher socioeconomic status were more likely to take up screening but with less DR detected. CONCLUSIONS: The inverse care law appears to operate in a preventive intervention even with a relatively small co-payment. There is a strong case for making effective preventive services free of charge.