A 9‐year‐old boy with short stature and abnormal genitalia was found to have an XY/XYY chromosomal constitution. On physical examination, in addition to his short stature he had a shield chest and external genitalia showing a hypospadias with cryptorchidism on the left side. The right testicle was present in the scrotum. His intellectual performance was normal and there wax no history of abnormal behavior. Surgical exploration revealed in the left side a streak gonad with a vas deferens and a fallopian tube. Fluorescencc studies of interphasic nuclei and chromosomes confirmed the XY/XYY karyotype. Two hypotheses are advanced to account for this mosaicism.