Familial Mediterranean fever (FMF) is a systemic relapsing autoinflammatory disorder occurring in populations originating from the Mediterranean basin, mainly Turks, Levantine Arabs, Sephardic Jews, Druze, and Armenians. The prevalence of FMF shows considerable geographical variation. In Turkey, the prevalence rates were reported as 0.0027-0.25%. This field survey was conducted in different regions to investigate the frequency of FMF in a northern province of Turkey. This study was conducted in 70 areas (12 urban and 58 rural) in the province of Tokat, which is in northern Turkey. The population of Tokat was reported to be 828,000 at the last census in Turkey in 2000, about 530,000 for individuals aged > 18 years (http://www.die.gov.tr). Mean age of 1,095 (541 male and 554 female; urban 555 and rural 540) subjects was 41 +/- A 17 (range 18-95 years). FMF frequency in this study was 1/123 (0.82%, 95 CI: 0.40-1.61). Mean age of patients were 27 years (20-41) and mean age of symptoms were 16.3 years (11-23). In conclusion, the frequency of FMF in this study was 1/123 (0.82%) which was higher than expected. This rate is the highest frequency of FMF reported from Turkey. Further large sample studies are needed to define to true prevalence of FMF in Turkey.