A spontaneous subcapital femoral neck fracture is an uncommon complication of a healed intertrochanteric hip fracture. To determine the etiology of this complication, 274 patients who had been treated for intertrochanteric hip fractures were followed up over an 8-year period from June 1988 to June 1996. We found 7 fractures of the subcapital femoral neck without a history of a trauma. The remaining 267 patients were defined as the control group. All seven patients with fractures of the subcapital femoral neck were women, and their average age at the time of compression hip screw (CHS) fixation was 83 years (range 74-92 years). The subcapital fractures occurred between 4 and 36 months after CHS fixation. The difference in the Singh index between the patient and control groups was statistically significant (P < 0.01). However, with regard to the position of the screw, spinal score, age, ambulatory ability, height, weight, and intertrochanteric fracture type (Jensen type), the differences between the patient and control groups were not statistically significant. Based on our findings, we conclude that a high grade of osteoporosis is the most important predisposing factor in the spontaneous development of a subcapital femoral neck fracture after a healed intertrochanteric hip fracture.