Chest pain is a hallmark symptom in patients with unstable angina pectoris (UAP). However, little is known regarding the prevalence of an atypical presentation among these patients and its relation to subsequent care. We examined the medical records of 4,167 randomly sampled Medicare patients hospitalized with unstable angina at 22 Alabama hospitals between 1993 and 1999. We defined typical presentation as (1) chest pain located substernally in the left or right chest, or (2) chest pain characterized as squeezing, tightness, aching, crushing, arm discomfort, dullness, fullness, heaviness, pressure, or pain aggravated by exercise or relieved with rest or nitroglycerin. Atypical presentation was defined as confirmed UAP without typical presentation. Among patients with confirmed UAP, 51.7% had atypical presentations. The most frequent symptoms associated with atypical presentation were dyspnea (69.4%), nausea (37.7%), diaphoresis (25.2%), syncope (10.6%), or pain in the arms (11.5%), epigastrium (8.1%), shoulder (7.4%), or neck (5.9%). Independent predictors of atypical presentation for patients with UAP were older age (odds ratio 1.09, 95% confidence interval 1.01 to 1.17/decade), history of dementia (odds ratio 1.49, 95% confidence interval 1.10 to 2.03), and absence of prior myocardial infarction, hypercholesterolemia, or family history of heart disease. Patients with atypical presentation received aspirin, heparin, and beta-blocker therapy less aggressively, but there was no difference in mortality. Thus, over half of Medicare patients with confirmed UAP had "atypical" presentations. National educational initiatives may need to redefine the classic presentation of UAP to include atypical presentations to ensure appropriate quality of care.