Studies of platelet survival time, ‘turnover’, sequestration with use of 51Cr‐labelled platelets as well as platelet size distribution as an index of the age of platelet population and the rate of platelet production were performed in 13 patients with cyanotic congenital heart disease (CCHD). Platelet survival was significantly shorter in CCHD than in the normal subjects. The platelet population was younger in CCHD and the size distribution of platelets showed a shift to the right, i.e. the percentage proportion of megathrombocytes was increased (24–52 %) as compared with controls (6–15 %). The absolute number of megathrombocytes was also increased in some patients. The differences from the control values were statistically significant. The results suggested that an increased peripheral destruction of platelets (thrombocytolytic state) is present in children with CCHD.