Antibody for HTLV-I in sera from 11,224 residents in Miyazaki Prefecture, Japan, was determined by indirect immunofluorescent antibody method to compare age- and sex-specific antibody prevalence among three geographically divided areas of the prefecture. There was a significant difference in the positive rate of older age groups among three areas, 9.0% for the northern part, 14.5% for the southern part and 8.4% for Miyazaki City, in spite of little variation in younger age groups. A marked rise of positive rate in the southern part at the age of 40th was observed, which suggests that changes of some conditions for mother-to-child transmission happened in the time of their birth. Six seroconversions were observed by the follow-up study for five years of the 971 residents. They were between 28 and 38 years of age, four men and two women. This may provide another reason for the increase in antibody positive rate by age in the adult. However, it could not be investigated if the seroconversion was caused by a horizontal transmission from their spouse. The possibility of the long latency of the virus in man as in the experimental animal may also have to be considered.