AIM: To identify the effects of interleukin (IL) -13 on retinal pigment epithelial (RPE) cells and the IL -13 level in aqueous humor of age-related macular degeneration (AMD) patients. METHODS: IL -13 levels in aqueous humor specimens from AMD patients were detected with enzyme-linked immunosorbent assay (ELISA). ARPE-19 cells were treated with 10 ng/mL IL -13 for 12, 24, and 48h. The cell proliferaton was evaluated by the MTS method. The mRNA and protein levels of alpha-SMA and ZO-1 were evaluated with quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot respectively. The expression of tumor necrosis factor-alpha (TNF-alpha), transforming growth factor-beta (TGF-beta)and vascular endothelial growth factor (VEGF) were assessed by ELISA. RESULTS: IL -13 levels in the aqueous humor of patients with AMD were significantly higher than those in the control (167.33 +/- 17.64 vs 27.12 +/- 5.65 pg/mL; P<0.01). In vitro, IL -13 of high concentrations (10, 15, and 20 ng/mL) inhibited ARPE-19 cell proliferation. a-SMA mRNA in ARPE-19 cell were increased (1.017 +/- 0.112 vs 1.476 +/- 0.168; P<0.001) and ZO-1 decreased (1.051 +/- 0.136 vs 0.702 +/- 0.069; P<0.001) after treated with 10 ng/mL IL -13 for 48h. The protein expression of a-SMA and ZO-1 also showed the same tendency (a-SMA: P=0.038; ZO-1: P=0.008). IL -13 significantly reduced the level of TNF-a (44.7 +/- 01.67 vs 31.79 +/- 3.53 pg/mL; P=0.005) at 48h, but the level of TGF-I32 was significantly increased from 34.44 +/- 2.92 to 57.61 +/- 6.31 pg/mL at 24h (P=0.004) and from 61.26 +/- 1.11 to 86.91 +/- 3.59 pg/mL at 48h (P<0.001). While expressions of VEGF didn't change after IL -13 treatment. CONCLUSION: IL -13 in vitro inhibit ARPE-19 cell proliferation and expression in the aqueous may be associated with AMD.