Children with Down syndrome (DS) have a 10- to 20-fold increased risk of developing leukemia, particularly acute megakaryocytic leukemia. Newborns with DS or trisomy 21 mosaicism may exhibit a particularly unique form of leukemia that historically has been associated with a high rate of spontaneous remission. This transient leukemia (TL) has been shown to be a clonal proliferation of blast cells exhibiting megakaryocytic features. Its true incidence remains to be determined. At presentation, many infants are clinically well with only an incidental finding of abnormal blood counts and circulating blasts in the peripheral blood. However, in approximately 20% of cases, the disease is severe and life-threatening, manifesting as hydrops faetalis, multiple effusions, and liver or multi-organ system failure resulting in death. Of those children who enter a spontaneous remission, 13–33% have been found to develop subsequent acute megakaryoblastic leukemia, usually within the first 3 years of life, which if left untreated is fatal. This unique TL of the DS newborn has been the subject of recent clinical cooperative group trials as well as many biological and genetic research efforts. We summarize here the known clinical, biological, and cytogenetic features of TL associated with DS. Pediatr Blood Cancer 2005;44:29–32. © 2004 Wiley-Liss, Inc.