The aim of the present study was to investigate the effect of sildenafil citrate (viagra) on retinal venous diameter in patients with age-related macular degeneration (AMD). We investigated 14 male patients in a double-masked, randomized, placebo-controlled, crossover study. In each subject, one eye with typical non-exudative AMD fundus features was studied. Each of the subjects received 100 mg dose of sildenafil or matching placebo on two separate study visits. Monochromatic fundus photographs were obtained in the study eye before dosing and then 30, 90, 180 and 300 min later. Measurements of the diameter of the major retinal veins from digitized negatives were carried out using "Vessel map" static vessel analysis program (IMEDOS GmbH, Weimar, Germany). Statistical analysis of the data comparing the effect of sildenafil and placebo on venous diameters was performed using analysis of variance (ANOVA) for repeated measures. An analysis of variance (ANOVA) comparing the effects of sildenafil citrate and placebo on retinal vein diameters showed a significant interaction between time and treatment (P = 0.03). In comparison to placebo, sildenafil citrate produced a statistically significant vasodilatation of major retinal veins of 4.7% at 90 min (P = 0.004), 5.5% at 180 min (P < 0.0001) and 5.8% at 300 min (P < 0.0001). At 30 min there was a 2.2% difference, which was not statistically significant (P=0.14). Our results suggest that in patients with age-related macular degeneration, sildenafil citrate (viagra) produces a statistically significant vasodilatation of major retinal veins that is similar to what has been reported in normal subjects. Whether this vasodilatation is associated with changes in retinal blood flow needs to be further investigated.