PURPOSE: To compare the retinal vessel diameters of healthy eyes and active central serous chorioretinopathy (CSC) eyes, and to evaluate possible effect of retinal vessel diameter alterations on the pathogenesis of CSC. METHODS: This retrospective study included 39 patients with CSC and 34 healthy individuals. Spectralis optical coherence tomography + HRA with an infrared reflectance image were used to evaluate structure of retinal vessels in the circular region around the optic disc. For each eye, vertical inner and outer diameters of the four major arteries and veins were measured using infrared reflectance images, and vessel wall thicknesses were also calculated based on inner and outer diameters. RESULTS: The 304 vessels of the 39 active CSC eyes and 266 vessels of the 34 healthy eyes were used in the analyses. The mean venous wall thickness in active CSC eyes was significantly thicker than that in healthy eyes (40.0 ± 4.9 vs. 33.5 ± 4.1 μm, P = 0.001), whereas the mean venous inner diameter in active CSC eyes was significantly narrower (52.5 ± 9.7 vs. 61.3 ± 8.1 μm, P = 0.001). Also, the mean venous outer diameter was wider in CSC eyes, albeit not significantly (131.1 ± 7.0 vs. 128.5 ± 8.4 μm, P = 0.074). CONCLUSION: Our results suggest that the alterations of retinal venous diameters may play a potential role in the pathogenesis of CSC in addition to alterations in choroidal thickness.