OBJECTIVE: Familial Mediterranean fever (FMF) is an autosomal recessive hereditary disease which primarily affects non-Ashkenazi Jews, Armenians, Arabs, and Turks. The gene responsible for the disease (MEFV/FMF) has been recently identified. Four common mutations in exon 10 of the MEFV gene seem to account for 86% of the DNA variations identified in patients with FMF. We conducted a phenotype/genotype correlation study in a mixed population of Jewish and Arab children with FMF. STUDY DESIGN: Seventy patients clinically diagnosed as having FMF underwent molecular genetic studies using polymerase chain reaction and restriction endonuclease digestion methods to detect the presence of the four mutations (M694V, M680I, V726A, M694I). We then correlated the presence of each mutation with ethnic origin, age of onset, clinical manifestations, disease severity, and occurrence of amyloidosis. RESULTS: The M694V mutation, which is predominant in non-Ashkenazi Jews, was found in 92% of our Jewish patients and in only 30% of the Arab patients. All four mutations were identified among 94% of the Arab patients, but with no particular prevalence for any one of them. The presence of a homozygous M694V mutation was significantly associated with a more severe form of the disease: the clinical onset of the disease manifested at an earlier age; the number of attacks per month was higher; the global assessment by the treating physician and the severity of pain scored higher; and arthritis was more frequent. Only patients with the M694V mutation had a family history of amyloidosis. No association was found between the type of mutation and the predominance of fever, abdominal pain, pleuritis, skin eruption, or response to colchicine in the clinical picture. CONCLUSIONS: Homozygosity for the M694V mutation, predominant among North African Jews, is associated with a severe course and prognosis for FMF. This mutation is less common among Arabs and, when present, occurs almost only in heterozygous form. In Arab patients, the disease tends to run a milder course and seems to bear a better prognosis. The phenotype/genotype patterns that are evident from our study of a mixed series of Jewish and Arab children with FMF might provide a rational basis for counseling about the natural history of the disease and for clinical treatment of FMF patients and their families.