We investigated platelet aggregation and secretion from dense and alpha-granules in vitro in 28 tension-type headache (TH) patients and 26 healthy controls. We also measured basal platelet serotonin levels. Platelet aggregation was normal in TH, but the secretion of serotonin and platelet factor 4 (PF4) was significantly increased in response to 0.5 and 2.0 micrograms/ml collagen and to 1.0 mumol/l PAF. The basal platelet serotonin levels were also higher in patients than in controls. The mechanisms of platelet hypersecretion remain to be determined, but the increased secretion of serotonin is probably in part related to the increased basal levels. The increased platelet serotonin in TH patients may reflect an enhanced serotonin turnover.