Osteoporosis has been classified into primary and secondary forms. All patients with osteoporosis should have measurements of 25-hydroxyvitamin D, serum and urine calcium, and some estimation of renal function. There are a wide variety of disorders that lead to secondary osteoporosis, and the tests that confirm these diagnoses are described herein. Making the specific diagnosis is important because treatment of the underlying condition may be sufficient to lessen fracture risk, although some patients may also need usual treatment for osteoporosis. Laboratory testing in addition to a careful history and physical examination will often lead to diagnoses of treatable conditions.