Platelet aggregation in response to 5-hydroxytryptamine (5-HT) was investigated in 110 patients with cardiovascular diseases and in 40 age-matched control subjects. It was found that 40% of those patients had a biphasic irreversible platelet aggregation in response to 5-HT, whereas 92.5% of the control subjects responded with a weak reversible aggregation. A double-blind, placebo-controlled study, including 41 patients, showed that a subacute treatment with ketanserin significantly abolished platelet aggregation in patients hyperreactive to 5-HT. In an additional open study, including 10 patients with peripheral arterial obstructive disease, chronic treatment with ketanserin for a period of 3 months significantly suppressed the primary platelet aggregation to 5-HT at 2 X 10(-5) M and at 2 X 10(-6) M and significantly lowered the plasma beta-thromboglobulin levels.