A 3-year-old girl developed central nervous system, bone and bone marrow metastases, and hypercalcaemia shortly after presentation with medulloblastoma. Cytogenetic studies of the involved bone marrow showed multiple abnormalities including iso(17q). This chromosome rearrangement has been reported in other cases of recurrent or disseminated medulloblastoma. More studies are required relating the karyotypes of medulloblastomas to long-term outcome to determine if the presence of iso(17q) is a prognostic factor in this malignancy.