A consecutive sample of 64 healthy adults (33 female and 31 male) were recruited at the University of Michigan Medical Center, Ann Arbor. Data were available for analysis on 57 subjects. The participants were asked to take a single daily dose of aspirin ranging from 0 to 640 mg. Adherence to the daily aspirin ingestion was measured by self-report and the Medication Event Monitoring System (MEMS, Aprex Corp, Fremont, Calif); adherence rate for the study population was 35%. The adherence rates for all dosing errors between self-report and Medication Event Monitoring System were significantly different (P = .002). There was no significant gender difference in adherence rates. Adherence to regular aspirin ingestion was poor in healthy, paid subjects despite explicit, written and verbal instructions. Patient self-report alone is not a reliable measure of adherence.