We report two related patients, presenting duplication 20p, with a characteristic phenotype including normal growth pattern, mental and psychomotor retardation, reduced motor coordination, poor language development, round face and prominent cheeks, vertebral and dental anomalies, and renal malformations. Familial chromosome analysis showed a balanced translocation t(20;21)(p11;q22) in three members of the family. These cases, together with those previously reported in the literature, allow us to make a better delineation of the duplication 20p syndrome, identifying more clearly the symptoms that must be considered as characteristic of this clinical picture.