The purpose of the present study was to evaluate the prevalence and prognosis of patients incidentally diagnosed with hypertrophic cardiomyopathy (HC). We studied 380 consecutive patients with HC (49.3 +/- 17.2 years; 65% men) for a median of 58 months (range 6 to 454). The patients were divided into 2 groups: those incidentally diagnosed from routine examination findings (precordial murmur and/or abnormal electrocardiographic findings) and those diagnosed either because of symptomatic status or by screening because of a family history of HC. Those patients who had been incidentally diagnosed constituted 29.2% of our study cohort. Although overall mortality did not differ between the 2 groups (p = 0.12), the patients diagnosed either because of symptoms or a family history tended to have at least a 4.5-fold greater risk of cardiovascular death (relative risk 4.5, 95% confidence interval 1.04 to 19.6, p = 0.04) and a 4.22 greater risk of sudden death (relative risk 4.22, 95% confidence interval 1.0 to 18.22, p = 0.04). Despite the greater sudden death mortality among the nonincidentally diagnosed patients, no statistically significant difference was found concerning the sudden death risk factor frequency (p = 0.96) between the 2 groups. In conclusion, the discrepancy between the low numbers of patients reported by published registries and the relatively high prevalence of the disease in the general population can be attributed to the large number of patients who remain asymptomatic, even throughout their life, awaiting an accidental diagnosis. Those patients with an incidental diagnosis have a more benign course, as shown by the total cardiovascular and composite sudden death mortality. A high level of awareness and suspicion for HC among physicians is essential for clinical recognition of such patients.