A three-and-a-half-year-old boy presented in the blastic phase of chronic myelocytic leukaemia (CML) with lymphoblastic infiltration of CNS and testes. The clinical signs and symptoms and also blood and bone marrow findings were otherwise compatible with the chronic phase of the disease, and none of the factors predictive of early transformation were present. Cytogenetic analysis revealed that the Ph1 chromosome, with no additional chromosomal abnormalities, was present in 85% of the bone marrow cells. Meningeal leukaemia is almost unknown in the chronic phase of CML. However, the incidence in the blastic phase may resemble the incidence of CNS leukaemia at diagnosis in children with acute leukaemias. Testicular involvement appears to be extremely rare even in the blastic phase of CML; the "true" incidence may, however, also resemble that of acute leukaemias. This raises the question of the need for testicular and meningeal surveillance and prophylaxis, at least during the blastic phase of CML. One should consider whether the simultaneous meningeal and testicular lymphoblastic leukaemia in this patient was the result of blastic transformation at two independent sites, or whether the testes were seeded from the meninges without identifiable spread to blood and bone marrow.