Hip involvement is uncommon in familial Mediterranean fever (FMF) and can result either from a process specific to this disease or from a coexisting chronic inflammatory joint disease, usually suggestive of ankylosing spondylitis (AS). We report ten cases of FMF with radiologically-documented inflammatory hip disease. Five patients had AS and one had juvenile idiopathic arthritis. There were six men and four women, with a mean age of 34.4 years +/- 17.6 (range, 15-70 years). Onset of the inflammatory hip disease occurred after bouts of acute hip symptoms in one of the patients with isolated FMF and after protracted hip arthritis in another; the two other patients had no history of hip symptoms. The HLA-B27 antigen was looked for in two of the five patients with FMF and AS, with negative results in both; another patient in this subgroup had severe ulcerative colitis. Total hip replacement or replacement of the acetabulum was required in six patients, including two with isolated FMF. Chronic joint disease has been estimated to contribute fewer than 5% of the joint manifestations in FMF. In previous studies, the hips and knees were affected in 75% of patients with chronic joint disease related to FMF. The association of FMF and AS (usually without the HLA-B27 antigen) has been well documented, although the pathogenic mechanisms that link these two conditions remain unknown.