Phenolic glycolipid I (PGL-I) is a Mycobacterium leprae-specific antigen and the antibodies to the antigen may suggest an M. leprae infection. To compare the M. leprae transmission among the populations, we compared the prevalence of anti-PGL-I IgM antibodies among household contacts and controls between Korea and the Philippines. In Korea (prevalence of leprosy--0.04: 1000), the prevalence of anti-PGL-I antibodies were 4.8% among controls and 8.0% among contacts, respectively. On the other hand, the seroprevalence rate was 10.8% among controls and 13.4% among contacts in the Philippines (prevalence of leprosy--0.70: 1000). Interestingly, a marked difference was noted in the prevalance of anti-PGL-I antibodies among children between the countries; 10-14% among children under 10 years old and 15-18% among those aged between 10 and 19 in the Philippines compared to 0% and 2.9-6.4% in Korea, respectively. This study, therefore suggests that a high prevalance of anti-PGL-I IgM antibodies among children may indicate an active transmission of M. leprae, resulting in a higher incidence of leprosy in the population.