Data from 547 patients with aplastic anaemia who received bone marrow transplants from HLA-identical siblings were analysed to determine factors associated with the risk of interstitial pneumonia (IPn). IPn developed in 92 patients (17%). 37% of cases were associated with cytomegalovirus infection and 22% with other organisms; in 41% of cases no organism was identified. The case fatality rate was 64%; the mortality rate due to IPn was 11%. In multivariate analysis, four factors were associated with an increased probability of interstitial pneumonia: use of methotrexate rather than cyclosporine after transplantation (relative risk, 2.8; P less than 0.0008); occurrence of moderate to severe acute graft-versus-host disease (relative risk, 2.2; P less than 0.002); inclusion of total body radiation in the pretransplant preparative regimen (relative risk 2.2, P less than 0.004); and patient age greater than 20 (relative risk 1.7, P less than 0.002). The probability of IPn ranged from 4% for patients with none of these adverse risk factors to 51% (relative risk of 13.4) for patients with all four. The incidence of IPn decreased significantly between 1978 and 1985, paralleling a decrease in the use of total body radiation pretransplant for immune suppression and methotrexate post-transplant for prophylaxis against graft-versus-host disease.