We present a case of a 16-year-old boy with gynecomastia and symptoms of delayed puberty (relatively small testes and penis), who attended the Endocrinology Clinic. Pubic hair development was normal. Basic hormonal blood tests showed a primary testicular lesion (hypergonadotropic hypogonadism). The result of karyotype examination showed female karyotype 46, XX. Based on those results the boy was diagnosed to be 46, XX male. A replacement testosterone therapy was administered. He stays in follow-up for gonad observation. The authors emphasize the possibility of establishing the diagnosis of a severe disorder belonging to the group of inappropriate sex differentiation of sex reversal type not earlier than in teenage adolescents, who present symptoms of delayed puberty. In such cases the main rule in establishing a final diagnosis is played by a physical examination with evaluation of sex development, as well as basic hormonal blood tests and karyotype result. Their correct interpretation is possible only by a physician who has reliable knowledge of the physiology of male sex determination.