A patient with systemic lupus erythematosus (lupus) is described, in whom stress fractures of both legs developed, accompanied by swelling of both ankles. The swelling of the right ankle associated with fracture mimicked lupus synovitis, but resolved with healing of the fracture. On the left side the swelling represented true lupus synovitis coexistent with the tibial stress fracture. This presentation posed a diagnostic dilemma. It is necessary to maintain vigilance in the diagnosis of these fractures in lupus. Case report A 51 year old white woman with a 24 year history of discoid skin lesions and a photosensitive malar rash spontaneously developed pain and swelling of the right ankle in May 1987. The pain was aggravated by walking. Radiographs taken at the time were normal. She was referred for rheumatological consultation a month later because of the persistence of her complaints. She had taken prednisone 15 mg every other day and chloroquine phosphate 250 mg a day for the discoid skin lesions. There was some evidence of osteoporosis on old x rays. Skin examination confirmed the presence of discoid skin lesions on her back. On musculoskeletal examination, stress pain was elicited in both the tibiotalar and the subtalar components of the right ankle joint, and an effusion was detected. Straw coloured fluid (2 ml) was aspirated from the right ankle; there were a few cells, no crystals, and cultures were negative. The initial impression was that of a synovitis, and the question of systemic lupus erythematosus was raised. Both rheumatoid factor and antinuclear factor were negative, anti-DNA antibodies were not detected, and serum complement concentrations were normal. The patient was treated with an intra-articular injec-