The phenotypic effects of trisomy of various segments of chromosome 8 were recognized through the analysis of 12 different patients: 5 mosaic cases of trisomy 8, 1 case of trisomy for the short arm and the proximal segment of the long arm, 2 cases of trisomy for a portion of the short arm, and 4 cases of trisomy for the terminal segment of the long arm. The 3 syndromes, trisomy 8, trisomy 8p and 8q proximal, and trisomy 8q terminal are defined. Three clinical signs are common to the 3 syndromes: vertebral anomalies, depression of the mesosternum, and bulging of the forehead. Different segments of chromosome 8 may carry genes affecting osseous growth. Trisomy 8p causes, in addition to severe mental deficiency, a thick nose, a large mouth and microcephaly. Other clinical signs can be assigned to 3 groups corresponding to the short arm, the proximal part, and the distal part of the long arm.