The incidence of acute lymphoblastic leukaemia (ALL) is described for both children and adults for the three major immunophenotypes: null, CD10-positive (CD+) which includes both common and pre-B types, and T-cell (including pre-T variants). The data are derived from a population-based specialist registry of leukaemias and lymphomas covering approximately one-half of England and Wales. Null ALL predominates in those under 1 year old and CD10+ ALL in the 1-7 year olds. There is a male excess at all ages for T-cell disease, which is particularly prominent in adolescents and young adults. The effect of socioeconomic levels is seen most clearly for CD10+ ALL in the childhood peak, where B-cell precursor disease occurs more frequently in areas of higher socioeconomic status.