Background Persons with ID most often have incomplete, contradictory and imprecise knowledge of sexuality and sexual intercourse itself. They are not provided with sufficient information on their own body and sexuality, and are often discouraged from and sanctioned for trying to sexually express themselves. Sexual abuse due to low sexual assertiveness is also common. Aim The principal aim of this study was to establish the presence or absence of sexual activity in adults with ID residing in institutional housing, as well as the level and structure of their knowledge of sexuality, their sexual assertiveness and preparedness to react in a sexually dangerous situation. Methods The sample consisted of 100 participants with ID residing in institutional housing. The instruments used included the General Sexual Knowledge Questionnaire, What-if test and Hulbert index of sexual assertiveness. Comparative statistics included coefficient of linear correlation and multiple regression analysis. Results The results showed that 82% of the participants are sexually active. Most participants admitted to sometimes having sexual intercourse against their wishes as well as to having difficulty asserting themselves. Their knowledge of pregnancy, contraception and sexually transmitted diseases was very low. Female participants and those that reported having sexual intercourse had more sexual knowledge and were also more sexually assertive. Conclusion Knowledge of sexuality and sexual assertiveness of persons with ID residing in institutional housing is very low. Additional information on sexuality is necessary, as well as support in learning to express their own desires and to deal with unwanted sexual activity.