Acute chest syndrome (ACS) is an important cause of morbidity and mortality in sickle cell disease (SCD). Previous studies reported conflicting pictures of ACS making therapeutic interventions difficult. The Cooperative Study of Sickle Cell Disease prospectively followed 3,751 patients enrolled from birth to 66 years of age for ACS. Data on presenting signs and symptoms, laboratory findings, and hospital course were collected. There were 1,722 ACS episodes in 939 patients. Young children (age 2 to 4 years) presented with fever and cough, a negative physical exam, and rarely had pain. Adults were often afebrile and complained of shortness of breath, chills, and severe pain. Upper lobe disease was more common in children; multilobe and lower lobe disease affected adults more often. Severe hypoxia occurred in 18% of adults tested and could not be predicted by examination or laboratory findings. Bacteremia was documented in 3.5% of episodes, but was strongly influenced by age (14% of infants and 1.8% of patients > 10 years). ACS was most common in winter with children having the most striking increase. Transfusion was used less frequently, but earlier in children. Young children were hospitalized for 5.4 days versus 9 days for adults. Fifty percent of adults had a pain event in the 2 weeks preceding ACS and children were more likely to have febrile events. The death rate was four times higher in adults than in children. Fatal cases generally developed rapid pulmonary failure and one third were associated with bacteremia. Age has a striking effect on the clinical picture of ACS. In children, ACS was milder and more likely due to infection, whereas in adults ACS was severe, associated with pain and had a higher mortality rate.