BACKGROUND: Vasodilatory therapy of Raynaud's phenomenon represents a difficult clinical problem because treatment often remains inefficient and may be not tolerated because of side effects. METHODS AND RESULTS: To investigate the effects of sildenafil on symptoms and capillary perfusion in patients with Raynaud's phenomenon, we performed a double-blinded, placebo-controlled, fixed-dose, crossover study in 16 patients with symptomatic secondary Raynaud's phenomenon resistant to vasodilatory therapy. Patients were treated with 50 mg sildenafil or placebo twice daily for 4 weeks. Symptoms were assessed by diary cards including a 10-point Raynaud's Condition Score. Capillary flow velocity was measured in digital nailfold capillaries by means of a laser Doppler anemometer. While taking sildenafil, the mean frequency of Raynaud attacks was significantly lower (35+/-14 versus 52+/-18, P=0.0064), the cumulative attack duration was significantly shorter (581+/-133 versus 1046+/-245 minutes, P=0.0038), and the mean Raynaud's Condition Score was significantly lower (2.2+/-0.4 versus 3.0+/-0.5, P=0.0386). Capillary blood flow velocity increased in each individual patient, and the mean capillary flow velocity of all patients more than quadrupled after treatment with sildenafil (0.53+/-0.09 versus 0.13+/-0.02 mm/s, P=0.0004). Two patients reported side effects leading to discontinuation of the study drug. CONCLUSIONS: Sildenafil is an effective and well-tolerated treatment in patients with Raynaud's phenomenon.