Background. Effective treatment for patients with acute myocardial infarction is limited by patient delay in seeking care. Inadequate knowledge of heart attack symptoms may prolong delay. An intervention designed to reduce delay was tested in the Rapid Early Action for Coronary Treatment (REACT) Community Trial. In this report, the impact on knowledge of heart attack symptoms is presented. Methods. Twenty communities were randomized to intervention or comparison status in a matched-pair design. Intervention strategies included community organization, public education, professional education, and patient education. The main outcome measures were based on information regarding knowledge of symptoms collected in a series of four random-digit-dialed telephone surveys. Results. Knowledge of REACT-targeted symptoms increased in intervention communities. No change was observed in comparison communities. The net effect was an increase of 0.44 REACT-targeted symptoms per individual (P < 0.001). The intervention effect was greater in ethnic minorities, persons with lower household incomes, and those with family or spouse history of heart attack (P < 0.05). Conclusions. The REACT intervention was modestly successful in increasing the general public's knowledge of the complex constellation of heart attack symptoms. The intervention program was somewhat more effective in reaching disadvantaged subgroups, including ethnic minorities and persons with lower income. Despite these successes, the post-intervention level of knowledge was suboptimal. © 2003 American Health Foundation and Elsevier Inc. All rights reserved.