We conducted a review of cancers in Down syndrome (DS), because solid tumors are poorly understood in DS. Cancers are in excess in this condition because of the 20-fold excess of leukemias, whereas malignant solid tumors seem to be globally underrepresented as compared with those in the general population. However, among these tumors, some tumors are in excess: lymphomas, gonadal and extragonadal germ cell tumors, and possibly retinoblastomas and pancreatic and bone tumors. Neoplasms in excess are seen earlier, sometimes in fetal life (leukemias and testicular germ cell tumors) or neonatally (leukemias and lymphoma) and affect mainly male subjects. There seems to exist an excess of rare karyotypes. Other tumors are underrepresented, particularly neuroblastomas and nephroblastomas, in young children, and perhaps common epithelial tumors in adults. These observations suggest that DS has a particular tumor profile, with some tissues more affected by malignant diseases (hematopoietic tissue and germ cells) and others that seem to be protected (central and peripheral nervous system, renal tissue, and epithelial tissues). The mechanism is mainly genetic, but differences in exposure to exogenous agents compared with the general population must be kept in mind. These findings are of interest for the management of these patients and early detection of cancers. Better knowledge of this tumor profile could help us to understand the mechanisms of carcinogenesis and should be compared to the current knowledge of genes on chromosome 21. Am. J. Med. Genet. 78:207–216, 1998. © 1998 Wiley-Liss, Inc.