Fish oils are the most widely used nonvitamin, nonmineral dietary supplements in the United States. They are not over-the-counter medications and are neither approved nor indicated for treating disease. Patient knowledge and patterns of fish oil use are not well defined. To determine cardiac patients' knowledge and patterns of fish oil use. One thousand consecutive patients admitted to an in-patient cardiology service (2015-2017) taking fish oil dietary supplements or prescription omega-3 fatty acids were asked to complete an anonymous questionnaire concerning product knowledge and use. A total of 711 (71%) patients completed the questionnaire. Primary reasons for use included general health (34%), heart health (28%), arthritis (9%), and lipid disorders (8%). Few patients (14%) were advised to take fish oil products by a health-care provider. Only 2.5% were taking prescription omega-3 fatty acids. Only 26% knew the active ingredient in their fish oil product. Supplements were purchased through a nonpharmacy retail seller by 81% of respondents. Most cardiac patients consuming fish oil dietary supplements do so without medical supervision and without knowledge of the active ingredients. As most patients obtain supplements outside of a pharmacy, opportunities to monitor and educate patients remain a major challenge.