Neither the bone-matrix theory of osteoporosis established by Albright in the 1940's nor the lack-of-calcium theory of the 1960's especially represented by Nordin, due to experimental tests and clinical results could be maintained. Here a new theory of osteoporosis is introduced, explaining osteoporosis not to be primarily a disfunction of calcium- or bone metabolism, but as a part-symptom of disfunction of the whole sustentaculum -bones, marrow, nucleus pulposus and musculation. Osteoporosis is predisposed by amyothenia respectively in activity and is initiated by reduced blood circulation of the sustentaculum. Certain relevant conclusions for prophylaxis and therapy of osteoporosis can be deduced without neglecting already the presently only effective therapy using sodium fluoride.