India with its 4 million cases of leprosy, accounts for one-third of the world's population of leprosy patients. One-fourth of them are below 15 years of age. We report a 5-year follow-up study of healthy children who were close contacts of leprosy patients, in order to: 1. detect subclinical infection and observe the development of overt disease by using the Fluorescent Leprosy Antibody Absorption Technique (FLA-ABS) and the lepromin test which assess the humoral and cell-mediated immunity (CMI), respectively; 2. evaluate the efficacy of dapsone as a chemoprophylactic agent in the 'at risk' contacts. Four-hundred-and-fifty-five healthy contacts were studied. Majority of the contacts of multibacillary patients (303) were FLA-ABS positive (75 percent) and lepromin negative (55 percent) showing that although most of them had been infected, the lepromin status was negative (P < 0.01). On the other hand, the majority of the contacts of paucibacillary patients (152) were lepromin positive (57 percent) (P < 0.05). Furthermore, only 61 percent of contacts of paucibacillary patients were FLA-ABS positive as compared to 75 percent of contacts of multibacillary patients demonstrating that the former had been exposed to a lesser quantum of infection (P < 0.05). On the basis of results of FLA-ABS and lepromin tests, these 455 contacts were classified into four groups, viz. Group I comprising children who were FLA-ABS positive and lepromin positive; Group II, who were FLA-ABS positive and lepromin negative; Group III, who were FLA-ABS negative and lepromin positive; and Group IV who were FLA-ABS negative and lepromin negative.(ABSTRACT TRUNCATED AT 250 WORDS)