An important line of research involves asking people with intellectual and developmental disability (IDD) to self-report their experiences and opinions. We analyzed the responsiveness of 11,391 adult users of IDD services to interview questions from Section 1 of the 2008-2009 National Core Indicators-Adult Consumer Survey (NCI-ACS). Proxy responses were not allowed for the selected questions. Overall, 62.1% of participants answered the questions and were rated by interviewers as understanding the questions and as responding consistently. Most participants responded in an all-or-none fashion, answering either all or most questions or few to none. Individuals with milder levels of IDD and with speech as their primary means of expression were more likely to answer the questions and provide a scoreable response. Interviewer ratings of interviewees’ answering questions, understanding of questions, and consistent responding were each related to responsiveness.