The incidence of infection in 86 consecutive patients having bone marrow transplantation for acute or chronic myeloid leukemia, in a protocol in which cyclosporine was the main immunosuppressant, was low. Severe bacterial infections were infrequent and mostly caused by gram-positive cocci but early bacterial infection was often associated with severe graft-versus-host disease. Fungal infections were prevented by nystatin and amphotericin thus avoiding the difficult combination of cyclosporine and ketaconazole. Viral infections were no more common than in other series but, in patients with mismatched grafts, they tended to be associated with neurological complications clinically diagnosed as encephalitis.