The serotonin receptor blocker ketanserin was given orally in a double blind crossover study to 10 patients with connective tissue disorders and Raynaud's phenomenon. Eight of the 10 patients improved clinically on ketanserin and none on placebo. Digital blood flow was assessed with laser Doppler flowmetry (LDF), photoplethysmography, and skin temperature measurements. Laser Doppler flowmetry was the most useful method, showing a significant reduction in recovery time after a standard cold provocation. Although the resting flow was not significantly improved, digital ulcers healed in four out of five patients, providing evidence of increased nutritive flow. The results of this study suggest that orally administered ketanserin may be an effective and well tolerated treatment for Raynaud's phenomenon associated with connective tissue disorders, especially scleroderma.