We have reviewed 11 patients with idiopathic transient osteoporosis of the hip; the six who were women all developed the condition during pregnancy. Both simultaneous and sequential bilateral involvement were seen, but biochemical studies were consistently normal and one synovial biopsy showed only non-specific inflammation. Radioisotope bone scans and CT scans were useful to aid diagnosis. Treatment by limiting weight-bearing relieved symptoms, and spontaneous resolution was paralleled by radiographic remineralisation, usually within a few months. One patient developed a stress fracture of the hip and other areas of transient osteoporosis. A hip involved by the condition should be protected from overloading until bone density has recovered.