The authors report on the usefulness and problems of a new surgical procedure--posterior corrective fusion using a double-trajectory technique (cortical bone trajectory technique combined with traditional trajectory technique) in a patient with degenerative lumbar scoliosis and osteoporosis--with the aim of achieving and maintaining complete correction. A 64-year-old woman with severe osteoporosis required decompression and posterior lumbar fusion. Teriparatide therapy had recently been initiated, but the impairment that she was experiencing in her activities of daily living was severe enough that surgery could not be delayed until teriparatide might show efficacy. We decided to employ the double-trajectory technique described in this report in order to achieve the most solid fixation. As of the 14-month follow-up evaluation, the patient's postoperative course had been uneventful and there had been no loss of correction. The authors suggest that the double-trajectory method is useful for posterior fusion in patients with severe osteoporosis.