Ten normal, healthy, nonsmoking volunteers participated in a double-masked, placebo-controlled one-day trial of 2,000 mg of oral pentoxifylline to determine the short-term effects of orally administered pentoxifylline on retinal circulation and rheologic properties of whole blood. Retinal capillary blood velocity and leukocyte density, whole blood viscosity, and whole blood filterability were measured before and after therapy by the blue-field entoptic phenomenon computer simulation, rotational viscometry, and micropore filtration techniques, respectively. Treatment with pentoxifylline for one day resulted in a significant improvement in retinal capillary blood flow velocity (P = .02) and in viscosity (P < .01), but not filterability, of whole blood. These results suggest that pentoxifylline may be useful in the treatment of early diabetic retinopathy.