Familial Mediterranean fever (FMF) is an autosomal recessive disease causing attacks of fever and serositis. The gene causing this disease, designated MEFV, was mapped to the short arm of chromosome 16, but has not yet been cloned. North African and Iraqi Jews constitute the two largest population groups suffering from the disease in Israel. In this report we compared the severity of the disease between these two populations. North African Jews were found to have a more severe disease manifested by an earlier age of onset, an increase in frequency and severity of joint involvement, a higher incidence of erysipelas-like erythema, and a higher dose of colchicine required to control symptoms. The involvement of additional genes, environmental factors, and different mutations in MEFV, may explain the clinical variation in disease severity between these two population groups.