A newborn male trisomic for 12p is compared with three other 12p trisomics already reported in the literature, as well as with three patients monosomic for 12p. A "type and countertype" opposition is observed for five characters: in the trisomy, turricephaly, shortness of the nose, protruding anthelix, wide palms, and increased LDH-B activity; in the monosomy, protruding occiput, large nose, hypoplasia of the anthelix, narrow palms, and decreased LDH-B activity.