Haemorrhagic complications, which occur in up to 35% of infants during extracorporeal membrane oxygenation (ECMO), often produce devastating sequelae. Although many complex factors interact to control haemostasis, platelet number and function has significant impact on the development of primary haemostasis. The optimum platelet count on ECMO, however, has not been defined. At our institution prior to August 1987, platelet counts were maintained at greater than 100,000/mm3. After August 1987, however, platelet counts of greater than 200,000/m3 were maintained. In a retrospective study, patients were randomly chosen from these two treatment periods: group 1--March 1986 to July 1987; and group 2--June 1988 to June 1989. The average platelet count, platelets administered, hours on ECMO, and bleeding complications were compared to each other and to the July 1992 ELSO Registry. There was a significant difference in average platelet counts between group 1 and group 2. However, the amount of platelets administered per kg per day was similar. There was a significant difference in overall bleeding complications between Group 2 (12%) and the ELSO Registry (35%) (p < 0.01). There was a trend towards decreased complications in all subgroups, although sample size precluded significance. We conclude that increasing platelet counts to greater than 200,000/mm3 decreases the overall bleeding complication rate. This advantage is achieved without a continuous need for increased platelet administration once the desired level is reached and without an increase in perfusion time, mechanical complications, or mortality.