A randomized controlled chemoprophylaxis trial was carried out in Madras city using 560 disease-free household child contacts of 264 multibacillary cases as study subjects. In the study, 13 cases were diagnosed among 280 contacts who received 3 injections of acedapsone at 10 weeks interval as against 30 cases among 280 contacts who had the same number of placebo injections, during the follow-up period of 225 weeks. The difference in the incidences in the two groups was statistically significant. (X2 6.45; P less than 0.02). The protection due to the limited duration of acedapsone prophylaxis was 56.7 percent. There were no cases of multi-bacillary leprosy in either group. The efficacy of prophylaxis was significant in male children over 9 years of age and female children in the age-group 1-8 years. The other prognostic factors like the infectivity status of the index cases in the household and the duration of exposure to them could have possibly influenced the effectiveness of prophylaxis in preventing progression from infection to clinical disease among the subjects studied. Their effects could not be assessed in this study.