A case of transient leukemoid reaction in a child with Down's syndrome with the presence of leukoblasts in the blood at birth is reported. The karyotype established on culture of lymphocytes and fibroblasts was characterized by a chromosomic formula 47, XX,21+. The karyotype established on day 13 of life on cultured bone marrow showed a trisomic 21 abnormal clone with 46 chromosomes resulting from a translocation of the long arms of chromosomes 5 and 7, which gave the chromosomic formula: 46, XX, -5, -7,t (5 qter leads to cen leads to 7 qter), +21. This clone was present in spontaneous blood mitoses. It disappeared on day 75 of life as well as the abnormal leukoblasts. The peculiarity of this case in due to the presence of an aneuploid clone and the association of a leukemoid reaction and dysmegakaryocytopoiesis both of which were transient.