The author reviews diagnostic pro-cedures available in osteoporosis, describing the radiological and histological criteria which he has defined in more detail elsewhere. He discusses the pathogenesis of osteoporosis in relation to the Albright hypothesis and puts forward certain arguments to suggest that the condition could be due to prolonged negative Ca balance. He has found the Ca intake reduced in osteoporosis, calcium absorption relatively normal and the excretion of Ca normal or high. This could explain a condition of prolonged negative Ca balance.