Familial Mediterranean fever (FMF) is an autosomal recessive recurrent episodic inflammatory disorder that occurs with high frequency in certain populations in the Mediterranean area. Using extended pedigree data of 90 FMF probands, we calculated the FMF gene frequency in various ethnic groups in Israel by analyzing the frequency in a total of 2,312 first cousins. The heterozygote frequencies were as follows: 1:4.9 (0.2 +/- 0.06) for the Libyan subgroup, 1:6.4 (0.16 +/- 0.03) for the other North African countries subgroup, 1:13.3 (0.07 +/- 0.04) for the Iraqi subgroup, 1:11.4 (0.09 +/- 0.06) for the Ashkenazic subgroup, and 1:29.4 (0.03 +/- 0.03) for the remaining ethnic groups. The observed number of affected parents and offspring of the probands was in agreement with the estimated gene frequency. Thus, the FMF gene frequency is very high in all Jewish ethnic groups in Israel, especially those originating in North African countries. This also explains the parent-to-off-spring transmission of FMF reported in North-African Jews.