Overwhelming infections caused by encapsulated bacteria, salmonella spp. and Plasmodium falciparum (in malarious areas) are an important cause of morbidity and death in patients with sickle cell disease. Bacterial infections afflicting these patients include fulminant meningitis and septicaemia caused by Str. pneumoniae and H. influenzae type b, and non-typhoid salmonellosis. Children less than five years of age are at greatest risk for meningitis and septicaemia, while salmonella osteomyelitis is probably common to all age groups. The most important contributing factors to this increased susceptibility to encapsulated bacteria are: a state of functional asplenia, an opsonophagocytic defect due to an abnormality of the alternative complement pathway, and a deficiency of specific circulating antibodies. Devitalisation of gut and bone due to repetitive vaso-occlusive crises, saturation of the macrophage system with red cell breakdown products of chronic haemolysis, and underlying splenic and hepatic dysfunction all predispose to salmonella infections. Seventy per cent of septicaemias and meningitis among under-fives with sickle cell disease is caused by Str. pneumoniae. Septicaemia frequently presents with sudden fever, few prodromal features, and a deceptive appearance of well-being, followed within hours by rapid relentless progression to shock and death. Adrenal haemorrhage is common, and mortality can be as high as 50 per cent, unless intravenous antibiotic, with or without steroid therapy, is promptly initiated. The clinical presentation of bacterial meningitis, its management and mortality follow the normal patterns, but recurrent meningitis and cerebrovascular morbidity are common in patients with sickle cell disease. An acute pulmonary involvement, indistinguishable from bacterial pneumonia (the 'chest syndrome') is the commonest single complication of sickle cell disease at any age. Str. pneumoniae is responsible for about half of the episodes. The protective values of the pneumococcal vaccine and long-term penicillin prophylaxis remain to be established in sickle cell disease. Over 70 per cent of haematogenous osteomyelitis in sickle cell disease is caused by salmonellae. The distinction from vaso-occlusive bone crisis is often difficult, but the presence of multiple, often symmetrical bone involvement, diaphyseal fissuring and involucrum should suggest osteomyelitis rather than bone infarction. Chloramphenicol remains the drug of choice and often has to be given in high doses for up to six weeks. The role of surgery is limited by the presence of multiple bone involvement and the known anaesthetic risks in this group.(ABSTRACT TRUNCATED AT 400 WORDS)