The literatures that reported the efficacy of combined administration of vitamin D(3) and vitamin K(2) for primary osteoporosis are reviewed. Combined administration of vitamin D(3) and vitamin K(2) potentially stimulates bone formation (mineralization) in patients with primary osteoporosis, resulting in an increase in lumbar bone mineral density. Combined administration of vitamin D(3) and vitamin K(2) seems to have the greatest effect on lumbar bone mineral density in the studies in which the mean age and years since menopause of the subjects were low, in contrast to administration of antiresorptive agents, bisphosphonates that has the greatest effects on bone mineral density and incidence of vertebral fractures in the studies in which the mean age and years since menopause are high, and the subjects have marked clinical evidence of osteoporosis. Further prospective, randomized, well-controlled study with long duration of observation is needed to confirm the efficacy of combined administration of vitamin D(3) and vitamin K(2) for primary osteoporosis.