A child with ambiguous genitalia at birth was studied over 17 years. Both a penis with hypospadias and a vagina was shown to be present. Laparotomy at the age of one year showed an uterus - on the right side no gonad, but a hypoplastic fallopian tube could be seen - on the left side a gonad, shown to be infantile testis and a normal fallopian tube. The sex of rearing changed during time: girl/boy/girl. At the age of 14 hormonal study showed a very raised FSH and LH, in spite of the presence of a testis producing testosterone causing progressive virilisation. The left testis was removed, it showed Sertoli cells and a few germinative cells in the tubules and Leydig cell hyperplasia. A biopsy from the place of the ovary on the right side showed ovarian stroma, no primordial follicles. This sort of hermaphroditism with a dysgenetic testis and a “streak gonad” is called mixed gonadal dysgenesis or asymmetrical gonadal dysgenesis. Chromosome analysis showed a mosaic of 71% 46 XY and 29% 45 XO (blood) cells. Further analysis on the patient and the parents made a structurally abnormal Y-chromosome, a chimera and mosaicism due to simple non-disjunction improbable. It is concluded, that this case of asymmetrical gonadal dysgenesis must be due to anaphase lag. The course and the sex of rearing is discussed.