Immunological (surface-marker) tests have been used to define four subgroups of acute lymphoblastic leukaemia (A.L.L.) in childhood: T-A.L.L., B-A.L.L., common-A.L.L., and null-A.L.L. A study of 94 children shows that the common-A.L.L. subgroup achieves a much longer duration of remission than T-A.L.L.; our findings also confirm the association of some clinical features with T-cell A.L.L. Within the common-A.L.L. subgroup, initial white-cell count correlates with prognosis.