In all, 1490 patients underwent splenectomy in Western Australia between 1971 and 1983, giving 7825 person years exposure. Thirty-three patients developed severe late postsplenectomy infection (septicaemia, meningitis or pneumococcal pneumonia requiring hospitalization) and three developed overwhelming postsplenectomy infection. The incidence and mortality rates of severe late postsplenectomy infection were 0.42 and 0.08 per 100 person years exposure respectively and for overwhelming postsplenectomy infection the incidence and mortality rates were 0.04 per 100 person years exposure. There were 628 splenectomies after trauma, giving 3922 person years exposure. Eight patients developed severe late postsplenectomy infection of whom one had overwhelming postsplenectomy infection. Following trauma, the incidence of severe late postsplenectomy infection was 0.21 per 100 person years exposure, with the incidence and mortality rates of overwhelming postsplenectomy infection being 0.03 per 100 person years exposure. Patients undergoing splenectomy have a 12.6-fold increased risk of developing late septicaemia compared with the general population. Splenectomy following trauma gives an 8.6-fold increased risk of late septicaemia. The majority of severe late postsplenectomy infections did not occur within the first 2 years and 42 per cent of severe late postsplenectomy infections occurred greater than 5 years after splenectomy. The low incidence of severe late postsplenectomy infection and overwhelming postsplenectomy infection makes statistical evaluation of the effectiveness of prophylactic antibiotics, vaccination and splenic repair most difficult.