Clinical, chromosomal and radiological findings are described in 3 unrelated patients with trisomy 8 mosaicism syndrome (T8ms), in 2 first cousins with trisomy 8q and in a patient with trisomy 8p. The phenotypic and cytogenetic findings, seen in the 6 patients and those noted in previous reports, support the hypothesis (Riccardi and Crandall, 1978) that most physical malformations seen in T8ms are associated with trisomy for the long arm of chromosome 8. [Some of the clinicial findings in chromosome 8 trisomy include agenesis of the corpus callosum, external ear canal agenesis, mental retardation and multiple congenital malformations.].