The epidemiology of subacute sclerosing panencephalitis (SSPE) has changed substantially since the introduction of measles vaccine. We studied the incidence of SSPE in Bulgaria based on cases admitted to the Child Neurology Clinic, University Hospital of Neurology and Psychiatry, Sofia, for a 25-year period (1978-2002). The SSPE incidence prior to and during the period of routine measles immunization was analyzed. SSPE was diagnosed in 40 children (29 males and 11 females, mean age 8.5 years), 28 from 1978 to 1984 (average 4 patients/year), and 12 from 1995 to 2002 (average 1.7 patients/year). Thirty-eight cases (95%) were non-immunized and had early measles infection. Age at onset of SSPE ranged from 8 to 11 years (52.5%) with a mean latent period of 7 years following measles infection. The increase in SSPE incidence (1995-2002) following a 10-year disease-free period (1985-1994) appears to be related to early measles infection (mean age 11 months) during the measles epidemic of 1991-1992. During the period 1995-2002, children had earlier measles infection (average 11 months) and earlier onset of SSPE (mean age 8.4 years) than in the period 1978-1984 (mean age at measles infection 18 months, and of SSPE onset 11.2 years). The SSPE incidence in Bulgaria during the 25-year period from 1978 to 2002 confirms the importance of early measles infection as a risk factor for SSPE, and the role of routine measles immunization in SSPE prevention.