During 1983-7 a clinical diagnosis of subacute sclerosing panencephalitis (SSPE) was confirmed by the detection of measles virus haemagglutination inhibiting antibody in the cerebrospinal fluid (CSF) in 81 subjects resident in Tamilnadu. The antibody titre (reciprocol of the end-point dilution) in the CSF ranged from 2 to 32 and in the sera from 8 to 2048. The CSF:serum ratios of titres were 1:4-1:64 in 80 cases and 1:128 in one case. The median age at onset of SSPE was 10 years and 97% of cases were diagnosed at stage 2 and beyond. Based on the geographic distribution of 72 cases in an estimated population of 8.4 million, the annual incidence of SSPE was calculated to be 2.14 per million population, or 4.3 cases per million children below 20 years. Assuming that only 10% of all cases would have reached the level of laboratory diagnosis, the incidence may be as high as 21 cases per million population.