The first step is the separation of “primary” or “idiopathic” from the “secondary” osteoporoses, which have an underlying cause, i.e. a specific disease or disorder. “Primary” osteoporosis refers mainly to postmenopausal and age-related involutional osteoporoses, in spite of the fact that a number of factors contributing to their pathogenesis are already known. Secondary osteoporoses are responsible for about 20% of all osteoporotic fractures (Table 39.1). Secondary causes of osteoporosis, i.e. as comorbidities, are also frequent in older patients who already have primary involutional osteoporosis. Physicians should consider the possible causes of secondary osteoporosis, particularly when patients present as follows: