We report on a 13-month-old boy with abnormalities consistent with mosaic trisomy 8 syndrome and male genitalia with partial penoscrotal transposition without hypospadias, a retractile left testis in inguinal canal, and an absent right testis. A voiding cystourethrogram showed an outpouching close to the lower right side of the bladder (utriculum) and bilateral hydronephrosis secondary to vesicoureteral reflux. Peripheral blood karyotype was 45,X/47,XY,+8. The karyotype of cultured skin fibroblasts was 47,XY,+8 with no 45,X cells detected among 20 cells counted. Tissues removed during surgery documented a 45,X/47,XY,+8 complement in the left testicle and utriculum, but only a 45,X line among 20 cells counted from vas deferens tissue. A possible mechanism for the origin of this previously unreported mosaicism might be an abnormal zygote with a 47,XY,+8 complement with subsequent simultaneous loss of chromosome Y and 8 in a cell at a very early embryonic stage.