Familial Mediterranean fever (FMF) is a recurrent episodic inflammatory disorder of unknown pathogenesis that occurs with high frequency in non-Ashkenazi Jews and Armenians. However, there are some differences in the clinical manifestations of FMF in these ethnic groups. FMF has been reported to be an autosomal recessive disease in non-Ashkenazi Jews, with a male/female ratio of 1.7, indicating reduced penetrance in females. However, the inheritance is less clear for Armenians. To resolve this problem, we studied prospectively families of 64 Armenian index cases randomly ascertained at the UCLA FMF clinic. Fifty-three families containing 176 sibs in addition to the probands were analyzed by genetic segregation analysis (exclusions included: six single-child families, four families in which one of the parents was also affected, and a family with incomplete information). Upper and lower bounds of the s e g regation ratio were estimated, and ranged from .10 f .03 to .18 & .05 when only definitely affected sibs were classified as affected; .17 f .04 to .27 f .05 when considering "possibly affected" sibs as affected; and .19 f .04 to .30 f .05 when incomplete penetrance in females was corrected. A value of .25 is the expected segregation ratio for autosomal recessive inheritance, and our data are consistent with this mode of inheritance. We can reject auto-soma1 dominant inheritance, where the expected segregation ratio is .5. Using extended pedigree data, we calculated an FMF gene