Plasma levels of beta-thromboglobulin (beta-TG), platelet factor 4(PF4) and platelet aggregation rate (PAR) were measured in remission phase of 15 patients affected with chronic pulmonary heart disease (CPHD). beta-TG, beta-TG/PF4, PF4 and PAR were significantly higher in the patients than in controls (P < 0.01 and 0.05, respectively). After 10 days of treatment with Dipyridamole 100mg tid, beta-TG, beta-TG/PF4 and PF4 decreased significantly compared with pretreatment values (P < 0.01 and 0.05, respectively). The results suggest that in vivo platelet activation is indeed present in patients with CPHD and that dipyridamole can antagonize platelet activation in vivo.