In dense and slum areas, parents often have to share bedroom with their children, so the children have been exposed to sexual activity since early. This study aimed to determine residential density, parents’ sexual activity and its effects to teenage sexual behavior in Yogyakarta. This study used cross-sectional design by interviewing 268 families that had teenagers as selected randomly in urban slum areas and rural areas on March – May 2015. Data was analyzed using chi-square test and multivariate cox-regression. Results showed that parents living in dense residence had risk two times higher to commit sexual activity which had negative effect on their children. Parents’ sexual activity did not have any influence to risky sexual behavior among teenagers. Factors significantly related to risky sexual behavior among teenagers are male sex, negative attitude and influence of peers. Health agency and primary health care should improve counseling programs for teenagers and train peer counselors. Activities which have been conducted at schools should be expanded to risky areas, such as urban slum area with high density of residence.