OBJECTIVE To evaluate the effect of adding testosterone undecanoate 40 mg daily to estrogen therapy on bone markers, bone mineral density and body composition in oophorectomized women. METHODS Fifty women, 45-60 years old, who had undergone a hysterectomy and bilateral salpingo-oophorectomy for benign disorders, were randomly assigned to oral treatment with testosterone undecanoate 40 mg plus estradiol valerate 2 mg daily or placebo plus estradiol valerate 2 mg daily. Twenty-four weeks later, cross-over was performed to the other treatment regimen. Forty-four women completed the study. Their serum concentrations of insulin-like growth factor (IGF)-I, IGF binding protein (IGFBP)-3, osteocalcin, carboxyterminal telopeptide aminoterminal (ICTP), of type I collagen propeptide of type I procollagen (PICP) and interleukin (IL)-1 receptor antagonist were measured at baseline and after 24 weeks of both treatments, as were also their body mass index (BMI) and blood pressure. Bone mineral density of the total body, spine and hip and total body fat, total lean body mass, trunk fat and trunk lean mass were determined by dual-energy X-ray absorptiometry measurements at baseline and after 24 weeks of both regimens. RESULTS During treatment, the addition of testosterone counteracted the decrease in IGF-I and PICP seen with estrogen therapy alone. Osteocalcin and ICTP were significantly reduced to the same extent by both therapies. No change ocurred in the IL-1 receptor antagonist. A significant increase was seen in total lean body mass with the estrogen/testosterone regimen, but the total fat mass, trunk lean or fat mass remained unchanged after 24 weeks of both treatments. No effect was detected on total, hip or spinal bone mineral density after treatment with estrogen alone or estrogen/testosterone. Likewise, BMI and blood pressure were unaffected. CONCLUSIONS The addition of testosterone to oral estrogen might have positive effects on bone as suggested by the fact that it counteracted the decline in IGF-I and PICP levels. An anabolic effect on muscle was reflected by an increase in the total lean body mass. No adverse effects were noted on BMI, fat distribution or blood pressure during the 6-month treatment with oral testosterone undecanoate.