This article covers in vitro, in vivo, and human data on the positive effect of vitamin K2 on osteoporosis. Data is available on vitamin K2 for osteoporosis caused by a number of conditions, including postmenopausal osteoporosis, Parkinson's disease, biliary cirrhosis, stroke, and drug-induced osteoporosis. The activity of vitamin K2 involves both an increase in the bone-building process and a separate decrease in the bone-loss process. Vitamin K2 exerts a more powerful influence on bone than vitamin K1, and should be considered for prevention or treatment in those conditions known to contribute to osteoporosis.