Ketanserin, 40 mg b.i.d., or matching placebo were administered for 8 weeks each in a randomised double-blind crossover design to 23 patients with Raynaud's phenomenon. Ketanserin had no effect on Doppler arterial patency or blood flow at rest, 37 degrees C, 15 degrees C, or during recovery after cold challenge. Red cell deformability index and whole blood viscosity were not significantly affected by ketanserin treatment. In vivo bleeding time was prolonged on ketanserin (p less than 0.05) but beta-thromboglobulin and platelet factor 4 were unaffected. There was a nonsignificant decrease in platelet aggregation response to serotonin but no change at all with other aggregating agents on treatment with ketanserin.