Clinical and genetic findings of familial Mediterranean fever (FMF) may be variable in different populations. Environmental factors may also affect phenotypic features of FMF. In this study, we investigated demographic, clinical and mutational features of FMF patients who were treated in a single reference hospital in Turkey. Two hundred and sixty patients (169 females, 91 males, mean age 30.44 +/- 10.29 years) were included in this study. All patients were evaluated regarding MEFV gene mutations. The mean age of disease onset was 17.21 +/- 8.66 years (range 2-40 years). The mean duration between the disease onset and diagnosis was 9.39 +/- 8.92 years. Seventy percent of patients had symptoms before 20 years of age (early onset FMF). Arthritis and erysipelas like erythema (ELE) were more common, and the mean duration between the disease onset and diagnosis was longer in early onset FMF patients. The frequency of attacks per year, and disease severity score (DSS) was higher in early onset patients. Homozygote mutation of M694V was detected in 37 (20.2%) and 4 (5.2%) patients in early onset FMF and adult onset FMF groups, respectively (p < 0.05). Histological diagnosis of amyloidosis was established in 7 patients (2.7%). The age of disease onset was earlier, and arthritis and ELE were more frequent, and DSS was higher in patients with M694V/M694V mutation. In conclusion, mean delay to diagnosis in our FMF population is quite high. Early and adult onset forms may differ regarding some clinical, molecular and prognostic characteristics. Disease activity was higher in patients with homozygote mutation of M694V.