The absence of CD10 (cALLA) in non-T non-B acute lymphoblastic leukaemia (ALL) is usually considered to be of adverse prognostic significance. From a large multicentre series of phenotyped ALL, we have identified a group of 23 non-T non-B ALL where blast cells were CD10 negative and CD19 positive. Class II antigens were present in 80% and C19 in 70%. Eight patients had successful karyotype analysis at diagnosis, and an additional patient at first relapse. Seven of these karyotypes showed a (4;11) (q21;q23) translocation. Most of the patients (70%) were young females, and they often presented with organomegaly. Six patients were less than 1 year old. The white cell count was over 100 x 10(9)/l in 48% of the cases. The FAB type was L2 in 56% of the patients. The most striking features were the poor response to therapy and survival. Six patients never attained complete remission and nine patients relapsed, most of them during the first year after diagnosis. Allogeneic bone marrow transplantation was performed in three children, of whom two are still alive 2 years after diagnosis. This study emphasizes the prognostic value of immuno-phenotypic and karyotypic investigations of ALL.