A high prevalence of human T-lymphotropic virus type I (HTLV-I) infection among Israeli Jews was previously reported. In the present study, screening for HTLV-I of Israeli Jews was expanded to 10 ethnic groups. HTLV-I antibodies were tested by the particle agglutination assay, ELISA, and by Western blot as a confirmatory method. The HTLV-I proviral genome was tested by nested PCR with tax primers (SK43/SK44 and Tr101/Tr102). The PCR tests were carried out in all seropositive subjects and the seronegative family members of the seropositives subjects in the Iranian population. Sixty-eight of the 1,679 subjects (4.1%) were found to be seropositive. The Jews originating from Mashhad had the highest infection rate of 60/306 (20%). Of the 479 Iranian non-Mashhadi Jews, 6 (1.3%) were seropositive. Of the 894 non-Iranian Israelis, only 2 (0.2%) were seropositive. HTLV-I proviral DNA was found in the peripheral blood lymphocytes of 66 out of 68 seropositive subjects and 6 out of 75 seronegative subjects. Sixty out of 123 (49%) Mashhadi Jews and 8 out of 14 (57%) non-Mashhadi Iranian Jews were PCR-positive. Three out of three seropositive non-Iranian Israelis were PCR positive. One non-Iranian Israeli (who originated from Ukraine) without family connections to the Iranian Jews was also PCR-positive. One hundred eighteen saliva samples (84 from subjects of Mashhadi origin, 31 from Iranian origin, and 4 of other origins) were also screened. Antibodies for HTLV-I were found in 23 out of 46 saliva samples from the individuals with particle agglutination (PA) and/or PCR-positive findings in blood. Twenty out of 23 PA-positive saliva samples also contained the proviral DNA. It is concluded that HTLV-I infection in Israel is mainly limited to Jews originating from Iran (most of them from Mashhad) and their family members.