Measles is a major cause of mortality and morbidity in children receiving treatment for leukaemia. A review was made of all the documented cases of measles in children in first remission from acute lymphoblastic leukaemia at four major treatment centres in 1974-84. Over the 11 years reviewed 1043 children with acute lymphoblastic leukaemia were referred to these centres. Fifty one (4.9%) died while in first remission and 15 (29.4%) of these deaths were due to measles or its complications: 12 cases of pneumonia, 10 of them fatal; and six cases of encephalitis, five of them fatal and the sixth child left severely handicapped. These children would have had at least a 50% chance of long term survival. The severity of measles in the immunocompromised patient reinforces the need to improve the poor uptake of measles immunisation in Britain.