[Transitory leukemoid reaction with regressive clonal course in a mongoloid newborn infant].

A neonate with Down's syndrome presented with partial blast infiltration that spontaneously regressed without relapse at age 6 months. Sequential cytogenetic studies of cells in blood and bone marrow initially showed presence of a clone with 46 chromosomes and reciprocal translocation of chromosomes 5 and 7. This clone and the hematologic abnormality later disappeared simultaneously. Two explanations may be suggested: a defect in the maturation of normal blasts due to abnormal environment in the bone marrow, or an intrinsic abnormality of hematopoietic cells. The case reported would favor the second hypothesis.