Acase of insufficiency fracture of the femoral neck that occurred during treatment for osteoporosis is reported. A 77-year-old woman (height 150 cm, body weight 43 kg) with osteoporosis associated with high bone turnover was treated with oral cyclical etidronate (400 mg/day for 2 weeks every 3 months). Three months after the treatment was started the patient experienced pain in the right hip joint while walking despite no evidence of trauma. Although radiographs were normal, weight-bearing was not possible because of pain. T2-weighted magnetic resonance (MR) imaging was used to detect a fracture line localized on the inferior aspect of the femoral neck. Because on bone marker measurement bone resorption was increased and bone formation was decreased from baseline, treatment was switched to oral alendronate (5 mg/day, daily). Pain resolved 3 weeks after the fracture was evident, and free gait was possible during the following 3 weeks. Follow-up radiographs, obtained 3 and 6 months after the fracture was evident, showed bony sclerosis on the aspect in which the fracture line was observed on the T2-weighted MR image. The dissociation (imbalance) of bone formation and resorption was also alleviated. A possibility of increased bone fragility should be kept in mind when oral cyclical etidronate is applied to elderly Japanese, small-physique women with osteoporosis at a daily dose of 400 mg (higher dose).