We describe 3 unrelated Japanese patients with familial Mediterranean fever (FMF) due to a compound heterozygous E148Q/M694I mutation in the MEFV gene. The first patient is a 38-year-old man who also has chronic myelogenous leukemia (CML). Because genomic DNA analysis of the patient's nail revealed the E148Q/M694I mutation, we concluded that the individual mutations were obtained congenitally. Interferon alpha therapy was effective against not only the CML but also the FMF. The second patient is a 42-year-old man with consanguineous parents and a 14-year history of recurrent lower abdominal and back pain associated with fever. He successfully responded to colchicine treatment. The third patient is a 23-year-old woman who has a family history of FMF and since the age of 11 years has had recurrent chest and abdominal pain with fever. The onset of FMF was at an early age in this case, in contrast with the late onset of the disease in the first 2 cases. This patient's mother also has a heterozygous M694I mutation and experienced the same symptoms until 30 years of age. Our data suggest that it should be recognized that there are more FMF patients in Japan than previously expected and that the frequency of the E148Q/M694I mutation may be significant in Japanese FMF patients.