Internet-based health research is increasing, and often offers financial incentives but fraudulent behavior by participants can result. Specifically, eligible or ineligible individuals may enter the study multiple times and receive undeserved financial compensation. We review past experiences and approaches to this problem and propose several new strategies. Researchers can detect and prevent Internet research fraud in four broad ways: (1) through the questionnaire/instrument (e.g., including certain questions in survey; and software for administering survey); (2) through participants' non-questionnaire data and seeking external validation (e.g., checking data for same email addresses, usernames, passwords, and/or fake addresses or phone numbers; (3) through computer information, (e.g., IP addresses and cookies), and 4) through study design (e.g., avoid lump sum compensation and interviewing participants). These approaches each have pros and cons, and raise ethical, legal, and logistical questions, given that ethical tensions can emerge between preserving the integrity of research vs. protecting the privacy and confidentiality of study respondents. While past discussions concerning the ethics of online research have tended to focus on the participants' ability to trust the researchers, needs now arise to examine researchers' abilities to trust the participants. This analysis has several critical implications for future practice, policy, and research.