Trisomy for the short arm of chromosome number 12 was diagnosed (by a G-banding method) in a girl with multiple congenital defects. Her mother and two sisters showed a balanced translocation 46,XX,rcp(12;21)(p11;p11), so, the affected girl was the result of a maternal adjacen t-1 meiotic segregation with a karyotype 46,XX,der21,rcp(12;12)(p11;q11)mat. Another sister decreased at 3 yr of age showed similar phenotypical features and was considered also affected although no karyotype studies were performed. Both affected cases were compared with a previous one and the concordant characteristics allowed the individualization of the following syndrome: severe mental retardation, peculiar flat facies with prominent checks, epicanthic folds, broad and irregular implantation of the eyebrows, broad and flat nasal bridge with short and narrow nose, anteverted nostrils and large philtrum, broad and prominent lower lip, low set ears with folded helix, prominent anthelix and deep concha, "spade" shape fingers (sharp-pointed distal phalanges) with shortness of the fifth, bilateral genu valgum, slightly increased space between first and second toes, secral dimple, generalized hypotonia and hyporeflexia of knees and ankles, nistagmus, retarded and dysrythmic bone age, simian creases or equivalent and distal axial triradii.