Co-expression of myeloid antigens on the leukaemic blast cells was evaluated in 532 children with a diagnosis of acute lymphoblastic leukaemia (ALL). Using a panel of monoclonal antibodies belonging to CD11b, CD13, CD14, CD15 and CD33 an overall incidence of 4.3% was found, with values ranging between 1.8% for CD14 and 6.1% for CD15. When the data were further dissected, a significantly higher incidence of co-expression was noted in null-ALL (15/70 cases = 21.4%), compared to cases expressing a more mature immunophenotype, i.e. common-ALL (7/394 cases = 1.7%) and T-ALL (1/68 cases = 1.4%) (P less than 0.001). In null-ALL, 9/15 patients were infants, five of whom with the t(4;11); two further children also had a t(4;11). The clinical outcome of the 23 cases which co-expressed myeloid antigens was unfavourable. Only two of the 15 null-ALL, two of the seven common-ALL and the unique case with T-ALL are in fact in persistent first remission between 19 and 93 months from diagnosis. Though the overall incidence of childhood ALL expressing myeloid antigens is low, the evidence that this co-expression may be related to an unfavourable clinical course and that it more frequently occurs in null-ALL, particularly in the first year of life, suggests that the routine assessment of myeloid antigens may allow to identify a subgroup of childhood ALL with a poor clinical outcome.