The exact incidence rate of subacute sclerosing panencephalitis (SSPE) in Turkey (and in Istanbul) is not known. We have conducted an active surveillance study to determine the epidemiological characteristics and the incidence rate of SSPE in Istanbul between the dates July 1, 2002 and July 1, 2004. We found that the incidence of SSPE in Istanbul is 2 per million. By logistic regression analysis, risk factors in SSPE development are determined as being at younger ages (OR: 1.199, 95%CI=1.047-1.372, P=0.009), living in crowded households (OR: 1.430, 95%CI=1.039-1.968, P=0.028), low education level of the mother (OR: 0.123, 95%CI=0.034-0.447, P=0.001), low household income (OR: 0.413, 95%CI=0.234-0.728, P=0.002), infant's being born out of Marmara region (Istanbul is in Marmara region of Turkey) (OR: 0.358, 95%CI: 0.172-0.746, P=0.006), infant's not being vaccinated against measles (OR: 0.495, 95%CI: 0.312-0.786), infant's having had measles before (OR: 0.235, 95%CI: 0.135-0.411). As a result, it is found in this study that SSPE is mostly related to having measles infection, and measles vaccination is found to be highly protective against SSPE. This is the first epidemiological study in SSPE from Turkey that conveys the incidence rate in Istanbul.