Platelet volume and total platelet mass are parameters of platelet activity and have become a prognostic factor in coronary heart disease. We have studied the influence of aspirin on platelet count, volume and total platelet mass in vitro and in a randomized double-blind, placebo-controlled study in 20 healthy young male volunteers in vivo. Aspirin had no influence in vitro on either platelet count, volume or mass. Repeated blood sampling during a 7-day treatment with 250 mg aspirin daily showed an increased platelet count (7.3% on day 1, 3.0% on day 2, 6.8% on day 4 and 9.3% on day 7; p < 0.01) and total platelet mass (7.2, 5.0, 8.6 and 11.5% on days 1, 2, 4 and 7, respectively, p < 0.01). Aspirin treatment without repeated blood withdrawal had no effect. These data indicate that aspirin may affect the circulating platelet mass under certain conditions.