The levels of IGF-I in the vitreous body and the intraocular fluid of patients with proliferative diabetic retinopathy (PDR) were determined using radioimmunoassay (RIA). Eleven vitreous specimens were obtained from the intraocular fluid of eyes of patients with PDR who underwent surgery during the operation. Eleven intraocular fluids from the same patients during reoperations were compared with controls. The expression of IGF-I mRNA in cultured human Muller glial cells was evaluated using reverse transcription-polymerase chain reaction (RT-PCR). The mean IGF-I level in the vitreous samples during initial PDR surgery and reoperation was significantly higher than that found in the vitreous of the control (p < 0.05). The level of IGF-I increased in 6 of the 11 cases. Cultured human Muller cells expressed IGF-I mRNA. The results indicate increased levels of IGF-I both in the initial vitreous and ocular fluid at post-operative re-proliferation. Muller cell is suggested as an origin of local IGF-I production.