In children with cyanotic congenital heart disease (CCHD) response of platelets to adrenaline was found to be abnormal in 30 of 54 patients, disaggregation with ADP occurred in 16 of 40, and collagen aggregation was diminished in 6 of 21. That these defects represented a platelet release abnormality (PRA) was shown by normal uptake of14C Serotonin in 25 of 26 patients but abnormal release in 18 of 26. The PRA was significantly more frequent in patients aged more than 45 months; with PCV 59%; Hb 19; and platelet count 175,000/μl. There was no correlation between the bleeding time and the PRA, suggesting that in CCHD the bleeding time is not an adequate test of haemostasis. Specific measurements of prothrombin were used to measure liver‐dependent coagulation factors. Prothrombin was significantly lowered in children with CCHD and, as with PRA, the deficiency was most severe in patients with polycythaemia and diminished platelet cou