The authors report a case of a dysgenetic male pseudohermaphroditism with a 45,X/46,XY karyotype in a mosaic form, which was diagnosed in an infant. The one-week-old infant was evaluated because of proximal hypospadias and retention of the right testis. The results of hormonal tests were the followings: serum FSH 5.2 mU/ml; LH: 2.0 mU/ml; testosterone: 144.3 ng/dl; androstendione: 0.42 µg/l; 17-hydroxyprogesterone: 1.12 ng/ml. Chromosomal analysis revealed 45,X/46,XY karyotype. Fluorescent in vitro hybridization showed that 51% of the lymphocytes had the Y chromosome and the SRY gene. Analysis of the SRY showed no deletion in the AZF a,b,c regions. Pelvic magnetic resonance imaging indicated the presence of vagina between the bladder and the rectum, and it showed a mass measuring 15×8 mm in the right inguinal canal as well as an oval gonadal mass with a size of 13×7 mm in the left scrotum. During surgical intervention, performed at the age of one, the right gonad was removed and biopsy of the scrotal testis was performed. Histological examination revealed dysgenetic testis in both sides. The authors emphasize the necessity of cytogenetic and endocrinological investigations of newborns with perineoscrotal hypospadia and bilateral or unilateral maldescent testes immediately after birth. Surgical removal of the dysgenetic testicular tissue located in the abdominal cavity and its histological evaluation provides separation of mixed gonadal dysgenesis, dysgenetic male pseudohermaphroditism, bilateral gonadal dysgenesis and ovotestis in the 45,X/46,XY mosaic cases. An accurate evaluation is necessary for a correct sex assignment and for surgical intervention to prevent neoplastic degeneration of the dysgenetic gonad.