To date only seven cases of naturally occuring inactivating mutations of the aromatase gene in men have been documented. Osteoporosis is one of the typical signs of the aromatase deficiency in male patients. We report results of estrogen therapy in patient with formerly described frameshift mutation of the CYP19 gene, ins 1058 T. Bone mineral density (BMD) was assessed during three phases of estrogen therapy. Patient was substituted by calcium and vitamin D throughout the study. Therapy started with t.d. estradiol (E2) at dose 50 μg daily two days in week (phase 1). BMD increased in all measured localisations (vertebral +16%, neck +5.5%, ultradist +11%). Due to mild increase of body weight (+3.6 kg) as well as worsening of insulin resistance (fasting insulinemia 167.0 mIU/l opposed to 27.5 mIU/l in phase 1) we reduced dose of t.d. E2 to 25 μg daily one day in week (phase 2). After this change, fasting insulinemia moderated to 58.9 mIU/l, however we registered fall of BMD in lumbar spine (vertebral −9%, neck +14%, ultradist +11%). Therefore dose of E2 was increased and considering the patients preference we started equivalent i.n. formulation of E2 2 injections of E2 hemihydrate (1 injection ≈ 0.07 ml contains 150 μg of E2 hemihydrate) weekly, i.e. 300 μg weekly equivalent to t.d. E2 in dose 50 μg daily one day in week. At this dose we noted divergent changes of BMD (assessment after phase 3, vertebral +8%, neck −7%, ultradist −13%). Although showing positive effect to BMD, general effect of E2 therapy (depending on dose, length, formulation) demonstrating as worsening of a priori presented metabolic syndrome, remains questionable.