Two infants with congenital nonlymphoblastic leukemia were discovered to have mosaicism for trisomy 21. Both infants achieved durable spontaneous remissions. Trisomy was apparently restricted to the leukemic clone and could be detected in neither phytohemagglutinin-stimulated peripheral blood cells or bone marrow in either patient nor in myeloid progenitor cells from the second patient after resolution of the transient myeloproliferative disorder. We conclude that spontaneous remission of congenital leukemia is not confined to infants with partial or complete systemic trisomy 21 but can occur in genetically normal newborns whose leukemic cells contain a third chromosome 21.