Bloom syndrome (BS) was diagnosed in a 7-year-old boy during hospitalization for acute lymphoblastic leukemia (ALL). The patient had most of the signs of BS along with some atypical manifestations: absence of telangiectases, obesity, and moderate mental retardation. Results of the cytogenetic studies were fully consistent with the diagnosis of BS: the occurrence of quadriradial figures and a very high incidence of sister-chromatid exchanges (SCE). This child's ALL was of non-B, non-T type with the presence, at the time of diagnosis, of a marrow clone including two markers. A Yq - chromosome was detected in about 10% of PHA-stimulated lymphocytes but neither in bone marrow cells nor in skin fibroblasts. This case is the fifth instance of ALL out of 104 registered cases of BS.