To examine whether the administration of 1 alpha (OH)-vitamin D3 (1 alpha (OH)D3) could prevent the occurrence of vertebral crush fractures in senile osteoporosis, crush fractures of the thoracic or lumbar spine in senile osteoporosis treated with 1 alpha (OH)D3, 1.0 microgram/day (22 cases), 1 alpha (OH)D3 1.0 microgram/day + Ca 1.0 g/day (16 cases) or Ca 1.0 g/day (23 cases) were compared with those in non-treated controls (25 cases). The average treatment periods were 1.7 +/- 0.7 years in the 1 alpha (OH)D3 group, 2.1 +/- 1.0 years in the 1 alpha (OH)D3 + Ca group and 1.7 +/- 0.9 years in the Ca group. Occurrence of spinal crush fractures per 1000 patient years was significantly less in the 1 alpha (OH)D3 + Ca group (P less than 0.01) than in the non-treated control group. These findings suggest that administration of 1 alpha (OH)D3 is effective in preventing the occurrence of pathological fractures in senile osteoporosis.