Familial Mediterranean fever (FMF) is an autosomal recessive disease characterized by recurrent inflammatory attacks of serosal membranes. Several studies have focused on the differences between frequency of the mutations and their phenotypical manifestations. The aim of this study was to evaluate whether or not this phenotypical variation is associated with the existence of particular mutations. Twelve MEFV (Mediterranean fever) gene mutations were investigated in 119 patients suffering from FMF. Heterozygote M694V (21/119), heterozygote E148Q (21/119), homozygote M694V (17/119) and heterozygote V726A (12/119) mutations were the most common mutations. Patients were grouped according to the presence of the M694V mutation: group I was M694V/M694V, group II was M694V/others, and group III was other/other. Mean severity scores for the groups were 13.94 +/- 4.10, 10.79 +/- 3.01 and 8.31 +/- 2.26, respectively. There were statistically significant differences between the mean severity scores of groups I and II (p = 0.029), groups I and III (p < 0.0001), and groups II and III (p < 0.0001). Diagnosis of amyloidosis was established in four (23%) patients of group I, and three (8%) patients of group II, but in none of the patients in group III. There was also a statistically significant difference between groups I and III (p = 0.046), but not between groups II and III (p = 0.083) and groups I and II (p = 0.317) in terms of amyloidosis development. In conclusion, we found a higher disease severity score and higher prevalence of amyloidosis in FMF patients who were M694V mutation carriers. Many ethnic groups live in Anatolia and more ethnic origin-based studies are needed to determine the real effect of these mutations on disease severity and amyloidosis.