46,XX/46,XY chimerism has previously been reported in patients with abnormal sexual development, and rarely in otherwise normal individuals. We report the first postnatally documented prenatal diagnosis of whole-body 46,XX/46,XY chimerism in humans, discovered by maternal age amniocentesis. The normal male phenotype in this child creates a dilemma in prenatal counselling, since genotypic male/female chimerism cannot be assumed to imply an abnormal sexual phenotype.