An observation on bacteriological conversion upon 56 bocteriological positives out of 131 cases of lepromatous leprosy treated by antileprosy morbile service was carried out. Those 131 lepromatous cases were a part of 335 total registered from three Guns of Gangjin, Hwa sun, and Goheung, Cheollanamdo, since 1957 to 1973 that has been under the control of antileprosy morbile unit. The results obtained are summalized as follows, 1. Lepromatous type was 36.9% of all cases. The ratio of male to female was 4.9 to 1. Age between 30-44, the most productive ages, occupied 38.0% and age over 60 occupied 18.0% of all. 2. 42.7% of lepromatous type was bacteriological positive. 43.1% was for male and 43.1% was for female. Age between 20 to 24 shown the highest positive at 63.7% and followed by aged over 60 (62.5%) and under 19 (60.0%). The lowest one was seen at age between 40 to 44 at 29.2%. 3. Bacteriological positive rate at registration was different in accordance with duration of treatment received befor registration. It came up to the highest 100.0% at the group treated one year before registration. That of the group not received any treatment was 56.4%, and the group received 4-5 years was the lowest at 26.1% 4. Bacteriological conversion rate was 44.6% and 14.3% of all positive casese regressed in bacterial number. 37.5% of all active cases converted bacteriologically within 5 years. 5. Bacteriological conversion rate in group received 1 year treatment before registattion was 66.6%. That in group not received any treatment was 31.8% and group received 4-5 years was 33.3%. 6. Considering of total duration reaseved treatment together with before and after registration, 33.9% of all active cases resulted bacteriological conversion or regression within 5 years.