Protrusio Acetabuli (PA) occurs in various rheumatic conditions and metabolic bone disorders as a result of bony remodeling in the pelvis. We sought evidence of early PA in patients with biopsy-proven postmenopausal osteoporosis and osteomalacia using a quantitative radiographic index of protrusion. The values in osteoporotic patients were not significantly different from those in controls and did not correlate with bone density measurements. In osteomalacia, the index was significantly increased relative both to the osteoporosis and control groups; early PA was found in 50% of patients with osteomalacia. This simple radiographic measurement may be useful in the differential diagnosis of osteopenia.