Rib infarcts and acute chest syndrome in sickle cell diseases. In the absence of evidence for pneumonia or pulmonary embolus, primary pulmonary infarction has been assumed to be the cause of the syndrome of chest pain, fever, and pulmonary infiltrate on chest X-ray that commonly complicates sickle cell anaemia. To find out whether the syndrome might be due to rib infarction, 99mTc-diphosphonate bone scans were done. In the eleven episodes thus investigated (10 patients) the scans showed segmental areas of increased radionuclide uptake in ribs, indicative of bone infarction. A possible sequence of events is that the rib infarcts are primary and cause bone pain, followed by soft tissue reaction, pleuritis, and splinting. The resultant hypoventilation leads to atelectasis and subsequent development of the radiographic changes of the acute chest syndrome. Prevention of hypoventilation and treatment of bone pain are important therapeutic goals. 