A group of 9 patients with pre-B cell acute lymphoblastic leukaemia (pre-B ALL) was identified prospectively within 209 patients with ALL. This variant of leukaemia was defined by the presence of heavy mu-chains in the cytoplasm of the malignant cells, and no surface immunoglobulins. Four patients displayed the CD10 (CALLA) antigen, in addition to cytoplasmic mu chains. A G-0 acute leukaemia (no blast cells in S-phase) was identified in two cases. Response to treatment of the patients was poor: Only four achieved complete remission; median survival was 24 weeks and the 40-week disease free survival was 20%. These figures are significantly worse than those obtained in patients with early-pre-B (CD10+) ALL. We conclude that the prevalence of pre-B ALL is low in our experience (4.6% of all patients with ALL) and that a poor outcome of treatment was related to it.