Despite public health initiatives targeting rapid action in response to symptoms of myocardial infarction (MI), people continue to delay in going to a hospital when experiencing these symptoms due to lack of recognition as cardiac-related. The objective of this research was to characterize lay individuals' knowledge of symptoms of acute myocardial infarction (AMI) and associated decision processes for timely action. Thirty participants were interviewed about their knowledge of AMI, then presented with unrelated, unfamiliar and familiar scenarios of AMI symptoms and instructed to "think aloud" as they made decisions in response to the scenarios in order to capture the decision process directly. Data were analyzed using qualitative and quantitative methods to identify the semantic relationships between knowledge and decisions. Results showed that most participants (80%) identified three symptoms or less (e.g., chest pain: 93%; dyspnea: 53%). All participants identified urgent actions (calling 911, going to ED) as the appropriate response to AMI symptoms. Urgent action decisions increased with familiarity of symptoms (57% for unrelated symptoms to 83% for most familiar symptoms), and was highest for the cardiac group. Lay knowledge of AMI is necessary, but not sufficient for people to develop required heuristics for timely action. This ineffective decision increases as a function of ambiguous and unfamiliar situations. Health education interventions should focus on teaching clusters of problems with varying levels of familiarity and complexity to increase flexibility in making decisions.