It has been generally accepted that the clinical onset of familial Mediterranean fever (FMF) begins before 20 years of age in most patients. In this study, we aimed to investigate the demographic and clinical characteristics of our FMF patients with an age of onset > or =20. Records of 401 patients (female/male: 204/197) that followed up between 1990 and 1999 were reviewed according to a pre-defined protocol. All patients fulfilled the diagnostic criteria of Livneh et al. The demographic and clinical features of adult-onset FMF patients were compared to those of patients with a disease onset before 20 years of age. There were 57 patients (14%) who experienced symptoms of FMF at > or =20 years of age; 34 of them (8.5%) reported their first attack between 20 and 29 years of age; 18 of them (4.5%) between 30 and 39 years of age and five patients (1.25%) had their first attack after 40 years of age. Arthritis (42 vs. 65%, p = 0.001) and erysipelas-like erythema (7 vs. 17%, p = 0.047) were significantly less frequent in patients with adult-onset FMF compared to patients with disease onset before 20 years of age. Arthritis and erysipelas-like erythema were less frequent in adult-onset patients compared to those with an earlier disease onset. Adult-onset FMF may be a form of disease with distinct clinical, demographic and molecular characteristics. Prospective clinical studies and investigation of genotypic features are needed to identify the characteristics of this phenotypic variant.