The clinico-pathological features of 42 children with autoimmune haemolysis are described. Over 65% of cases were seen before their 5th birthday. In this group males predominated by the ratio of 2.5:1, but in the older children both sexes were equally affected. The incidence decreased from 1 in 188 X 10(3) in young males to 1 in 1,780 X 10(3) in children over 10. Cases were classified serologically. Of particular note was the frequency of Donath-Landsteiner haemolysis which equalled that due to warm autoantibodies; together these groups made up 79% of the total cases. Most haemolytic episodes followed an acute infection. This was frequently mild and often involved the upper respiratory tract; in only 2 patients was haemolysis associated with underlying collagenosis. Typically there was a sudden onset of pallor and malaise; jaundice, splenomegaly and hepatomegaly were found in about half the subjects. Haemoglobinuria was characteristic of Donath-Landsteiner haemolysis. The illness was severe, with Hb levels falling below 6.0 g/dl in 28 patients. Prednisolone, blood transfusion and, where indicated, antibiotics were usually effective in treating the illness, with splenectomy reserved for cases where this treatment was unsatisfactory. In several individuals no treatment was required. Recovery was rapid, and complete recovery occurred in 83% of patients, usually within 6 months. Although 2 patients died, a generally optimistic prognosis can be given, particularly in the absence of an underlying chronic disorder.