25 consecutive episodes of acute chest syndrome in 13 adult patients with sickle-cell disease were studied retrospectively. Chest symptoms were present on admission in 23 of 25 episodes. Abnormal chest signs and an abnormal chest X-ray were present on admission in only 11 and 9 episodes, respectively, but developed later in the remainder. The 9 episodes with bilateral radiological changes were associated with higher pulse rates, longer-lasting fever, more profound arterial hypoxaemia, and greater falls in haemoglobin than the 16 unilateral episodes. 2 patients with bilateral disease died; both had platelet counts <100x 10⁹/litre. In 12 episodes (6 bilateral, 6 unilateral) exchange transfusion was required and produced striking improvement in 11. Despite intensive microbiological investigation, infection was found in only 2 episodes—1 mycoplasma and 1 evidence of Escherichia coli.Pulmonary intravascular sickling may account for much of the clinical picture.