The purpose of the study was to determine which category of hydrodynamic phenomena the smokestack in central serous chorioretinopathy (CSC) most likely belongs to: leakage by diffusion or bulk flow.Fluorescein angiograms of 13 eyes of 13 patients were reviewed and analyzed quantitatively. Two methods were used to assess the rate of fluid leakage. One was based on observation of the expansion rate of the bubble of stained fluid seen in the earliest phase of the angiogram, and the other one compared the area of the source of the leakage to the remaining area of retinal pigment epithelium (RPE) exposed to subretinal fluid, by using a standard value for RPE fluid resorption capacity per unit surface area and assuming that resorption equals leakage.The mean rates of leakage were 16.2 μL/mm(2)/h (95% CI, 11.9-22.1) with the expanding-bubble method and 16.1 μL/mm(2)/h (95% CI, 12.0-21.7) with the area-of-resorption method (P = 0.95, linear correlation r = 0.94). The repeatability coefficient for both methods was 36.3%.The study demonstrated sufficient overall agreement between the two methods of assessing leakage rates in smokestack CSC, with adequate repeatability. Leakage rates of the RPE lesions in smokestack CSC occurred at rates consistent with bulk fluid flow, rather than secretion and diffusion, indicating that the primary source of leaking fluid was not the RPE, but a segment of underlying choroidal vasculature.