Idiopathic osteoporosis is a frequent cause of osteoporosis in young premenopausal women. However, there are no data about the treatment of these patients. The aim of this study was to analyse the evolution of bone mineral density (BMD) in premenopausal women with idiopathic osteoporosis treated with a conservative approach. Retrospective study of 16 premenopausal women with idiopathic osteoporosis (aged 35.7+/-7 years) with a mean follow-up period of 3 years (1-6 years). BMD measurements at the lumbar spine and femoral neck were obtained in all patients at baseline and yearly (patients had one or more fragility fractures and/or a Z score < -2 in the lumbar spine or femur). Secondary causes of osteoporosis were excluded in all patients. Patients were treated with calcium and vitamin D to achieve a calcium intake of up to 1,500 mg/day and were advised to increase physical activity. A significant increase in lumbar and femoral BMD was observed after 2 and 3 years of follow-up, respectively (1.9+/-1.9% mean increase in lumbar spine, p= 0.021, at 2 years) (5.6+/-4.5% mean increase in femur, p=0.04, at 3 years). The serum total alkaline phosphatase (TAP) values increased at 2 years (122+/-46 vs 140+/-36 U/l, p=0.054). In addition, a negative correlation between baseline TAP serum values and lumbar BMD evolution at 2 years was observed (r=-0.748, p=0.013). No patient developed new skeletal fractures during the follow-up period. In young premenopausal women with idiopathic osteoporosis the conservative treatment with supplements of calcium and vitamin D associated with an increase of physical activity is associated with an increase in BMD without evidence of further skeletal fractures after more than 3 years of follow-up.