OBJECTIVE To study whether aqueous humor (AH) and plasma adiponectin (APN) levels were altered in proliferative diabetic retinopathy (PDR). METHODS After excluding subjects with hypertension, hyperlipidaemia, nephropathy, coronary heart disease, heart and renal failure, 20 consecutive type 2 diabetes mellitus (DM) PDR patients and 20 consecutive age-matched non-diabetic and senile cataract (non-DM) controls were enrolled in this study. AH APN and plasma APN concentrations were determined by enzyme-linked immunosorbent assay (ELISA). The expression of AdipoR1/R2 mRNA in peripheral blood mononuclear cells (PBMCs) was measured by real-time polymerase-chain reaction (PCR). The correlations between two parameters, including plasma and AH APN, AdipoR1/R2, plasma insulin and plasma glucose, were analyzed by Pearson's correlation. RESULTS In non-DM subjects, the APN concentration in AH was 1.26 ± 0.56 ng/ml, approximately 1/2500 of that in plasma (3.15 ± 1.36 μg/ml). The averaged AH APN concentration in PDR patients was significant higher than that of the non-DM controls (5.29 ± 4.09 ng/ml vs. 1.26 ± 0.56 ng/ml, p < 0.001). Plasma total APN was not correlated with AH APN in the PDR patients, controls or all subjects. No significant difference was observed in plasma total APN levels and AdipoR1/R2 mRNA levels in PBMCs between the PDR patients and the controls (p > 0.05). CONCLUSIONS Differences in APN concentration between AH and plasma suggest blood-retinal barrier (BRB) plays a key role in maintaining intraocular APN homeostasis. The high APN level in AH may be associated with PDR.