Background: There is a lack of longitudinal large-scale studies of sexual abuse in intellectual disability services. Such studies offer opportunities to examine patterns in disclosure, investigation and outcomes, and to report on incidence and trends. Methods: All allegations of sexual abuse (n = 250) involving service users as victims or perpetrators of sexual abuse over a 15-year period in a large Irish community-based service were analysed based on the data extracted from extensive contemporaneous case notes. Results: Victims or families were the most common concern raisers of abuse. Following multidisciplinary investigation, almost half (47%) of all allegations of sexual abuse were confirmed (n = 118). In confirmed episodes, more than half the perpetrators were adolescents and adults with intellectual disabilities, while almost a quarter were relatives. The most common type of abuse was sexual touch, although 31% of episodes involved penetration or attempted penetration. The most common location was the family home, followed by the day service and public places. A notable feature was the variation in the incidence of abuse over the study period, largely caused by episodes of multiple abuse. Conclusions: The incidence of confirmed episodes of sexual abuse of adults with intellectual disabilities may be higher than previously estimated. There is an urgent need for statutory guidelines, which require reporting of adult abuse, and provide protection for "bona fide" whistle blowers, similar to existing child protection legislation.