The effect of brief 37 degrees C pretransfusion incubation of apheresis platelets (n = 25) was studied in 14 allogenic bone marrow transplant recipients (5 children, 9 adults). Apheresis platelets were collected on a cell separator, divided in two aliquots, and stored 1 to 5 days at 22 degrees C during agitation. One aliquot was incubated at 37 degrees C for 1 hour before transfusion, and the other was transfused as a paired control. When the patient's peripheral platelet count fell below 30 x 10(9) per L, one aliquot was transfused. The other aliquot was transfused the following day. Each patient received the two aliquots in random order. Corrected count increment (CCI) at 1 hour after transfusion of 37 degrees C incubated platelets was 12.2 +/- 9.5 (mean +/- SD) compared to 7.5 +/- 5.4 for paired control platelets (p < 0.05). CCIs the next day were 4.4 +/- 6.1 and 2.4 +/- 2.7, respectively (not significant). It can be concluded that 37 degrees C incubation of apheresis platelets improves posttransfusion CCI.