An infant born at 38 weeks' gestation with ambiguous genitalia had a prenatal 45X karyotype but an enlarged phallus on an ultrasound scan at 31 weeks' gestation. The newborn examination demonstrated penoscrotal hypospadias with chordee and two gonads palpable in the scrotum with a right hydrocele. Ultrasound showed a saccular structure containing debris behind the bladder. The postnatal karyotype was revealed to be 45X/46XY, with a pseudodicentric Y chromosome. Cystoscopy/genitography identified a uterus and a right fallopian tube, which were removed along with a dysgenetic right gonad. Biopsy of the descended left gonad revealed rare germ cells. The final diagnosis was 45X/46XY male pseudohermaphroditism with testicular dysgenesis. One should be aware of possible chromosomal mosaicism and combine the prenatal karyotype with the ultrasound genital findings to formulate an intersex differential diagnosis.