• Reports in the 1970s suggested that acute chest syndrome (ACS) in children with sickle cell disease is usually due to bacterial infection. Studies in adults and more recently in children, however, showed that proved bacterial infection occurs in a minority of these patients and that vascular occlusion is the main pathologic process. In a retrospective study of 32 episodes of ACS in children, a definite bacterial infection was found in 3% (one patient), possible bacterial infection in 11% (four patients), and a possible mycoplasma in 13% (five patients). With the intent to dilute sickle cells, 23 patients received blood transfusion within 24 hours after hospital admission; all showed a dramatic clinical and roentgenographic improvement. Of the nine patients who did not receive a transfusion after hospital admission, the conditions of five patients deteriorated but improved after "late" transfusion; three patients showed slow improvement, and only one patient improved within 48 hours. From this we conclude that vascular occlusion might be the main process In ACS and that early blood transfusion may be valuable in shortening the course and decreasing mortality. The low hemoglobin value at presentation in our patients makes dilution of sickle cells possible by packed red blood cell transfusion rather than exchange transfusion.