Most experimental studies examining the use of pre-interview instructions (ground rules) show that children say “I don't know” more often when they have been encouraged to do so when appropriate. However, children's “don't know” responses have not been studied in more applied contexts, such as in investigative interviews. In the present study, 76 transcripts of investigative interviews with allegedly abused children revealed patterns of “don't know” responding, as well as interviewers’ reactions to these responses. Instructions to say “I don't know” when appropriate did not affect the frequency with which children gave these responses. Interviewers rejected “don't know” responses nearly 30% of the time, and typically continued to ask about the same topic using more risky questions. Children often answered these follow-up questions even though they had previously indicated that they lacked the requested information. There was no evidence that “don't know” responses indicated reluctance to talk about abuse. Implications for forensic interviewers are discussed.