Researchers who create and/or analyze data from longitudinal surveys nearly always assume that respondents' attributes are not changed as a result of being measured. Yet, research in psychology, political science, and elsewhere suggests that this assumption is not always warranted. Using an experiment embedded within a well-known web-based panel, we assess (1) the magnitude of panel conditioning on survey questions about illicit behaviors; and (2) the degree to which panel conditioning biases depend on the length of time between survey waves. We find evidence that answering questions about theft and drunk driving affects subsequent responses to those same questions, but only when baseline and follow-up surveys are relatively close together in time. We conclude by discussing the implications of our findings for survey design and for social scientists who use longitudinal data.