Although hypertrophic cardiomyopathy (HC) occurs in 1 of 500 adults, most cardiology practices treat relatively few patients with HC, suggesting that many affected patients evade clinical recognition. Determining the clinical circumstances under which HC is identified will provide clues to its under-recognition. Clinical triggers leading to diagnostic echocardiograms were analyzed in 711 consecutive patients with HC. In most (384 [54%]), HC was initially suspected only after the onset of cardiac symptoms or acute cardiac events. In a substantial minority (327 [46%]), HC was recognized while patients were asymptomatic, including 225 (32%) by routine medical evaluations, in 27 of whom (4%) HC was recognized during preparticipation examinations for competitive sports or other activities. Women, older patients (age > or =50 years), and those with outflow obstruction at rest (gradient > or =30 mm Hg) were more likely suspected to have HC by virtue of cardiac symptoms or events (p <0.0001). Conversely, patients with extreme hypertrophy (wall thickness > or =30 mm) and those at high risk for sudden death were more often asymptomatic and identified by routine or family screenings (p <0.0001 and p = 0.004, respectively). Patients who subsequently died of heart failure or experienced embolic stroke were more often identified by virtue of symptoms or acute events (p = 0.03). In conclusion, although most patients with HC were recognized clinically only after overt disease manifestations, a substantial minority were diagnosed by routine examinations while asymptomatic, including an important subset of patients with HC recognized solely because of findings on sports preparticipation screening. These data underscore the need for heightened awareness and clinical suspicion of HC to increase the number of diagnosed patients, including many who may be at high risk for sudden death.