PURPOSE To determine the prevalence of secondary causes of bone loss among patients with breast cancer with osteopenia and osteoporosis. PATIENTS AND METHODS All women referred to a bone health clinic over a 6-year period for bone evaluation were included in this retrospective study and stratified based on presence or absence of a breast cancer history. The prevalence of secondary causes of bone loss in the two groups was compared. RESULTS Of the 238 women identified, 64 women had breast cancer. The non-breast cancer group (n = 174) was significantly older (P = .015), had a lower mean weight (P = .019), lower 25 hydroxy-vitamin D level (P = .019), and greater degree of bone loss in both the spine and hip (P < .001 and 0.004, respectively). The presence of at least one secondary cause of bone loss, excluding cancer-related therapies, was seen in 78% of the breast cancer patient group and in 77% of the non-breast cancer group (P = not significant). Newly diagnosed metabolic bone disorders were seen in 58% of the breast cancer population. The most common was vitamin D deficiency, seen in 38% of patients in the breast cancer group and 51% of patients in the non-breast cancer group. Idiopathic hypercalciuria was diagnosed in 15.6%, primary hyperparathyroidism in 1.6%, and normocalcemic hyperparathyroidism in 3.1% of the breast cancer population. CONCLUSION A high prevalence of secondary causes of bone loss among patients with breast cancer supports a comprehensive evaluation in these patients, particularly those considering therapy with an aromatase inhibitor.