A 51-year-old woman developed skeletal symptoms due to histologically proved osteomalacia. The etiology could not be determined until a history of factitial diarrhea due to long-term phenolphthalein (Ex-Lax) ingestion was finally elicited. Normal bowel habits returned after the use of phenolphthalein was discontinued and the osteomalacia showed signs of healing. Surreptitious ingestion of cathartics should be considered as a causal factor in patients with unexplained osteomalacia.