A random sample of 409 comprehensive school classes with about 9,000 15-year-olds was selected to fill out questionnaires anonymously. The majority (93 percent) responded in the privacy of the school nurse's office, while the rest responded in their classrooms. Of the selected classes, 89 percent participated in the survey and the response rate of students in those classes was 96 percent. About 0.5 percent of girls reported sexual experiences with their father or stepfather, 0.2 percent of girls reported sexual experiences with their biological father, and 3.7 percent of girls living with a stepfather reported sexual experiences with him. Feelings of girls about their incestual experiences were overwhelmingly negative. Results of the Finnish survey are compared with prevalence estimates from the United States and Nordic countries. 19 references and 4 tables