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The prognostic significance of syncope has not been investigated systematically in hypertrophic cardiomyopathy, and treatment strategies have been based largely on intuition and experience.We assessed the relationship between syncope and sudden death in 1511 consecutive patients with hypertrophic cardiomyopathy. Unexplained (n=153) or neurally mediated (n=52) syncope occurred in 205 patients (14%). Over a 5.6±5.2-year follow-up, 74 patients died suddenly. Relative risk of sudden death was 1.78 (95% confidence interval 0.88 to 3.51, P=0.08) in patients with unexplained syncope and 0.91 (95% confidence interval 0.00 to 3.83, P=1.0) in those with neurally mediated syncope compared with patients without syncope. In multivariable analysis, the temporal proximity of unexplained syncope to initial patient evaluation was independently associated with risk of sudden death (P=0.006). Patients with unexplained syncope within 6 months before the initial evaluation showed a 5-fold increase in risk compared with patients without syncope (adjusted hazard ratio 4.89, 95% confidence interval 2.19 to 10.94), a relationship that was maintained throughout all age groups (<18, 18 to 39, and ≥40 years). Older patients (≥40 years of age) with remote episodes of syncope (>5 years before initial evaluation) did not show an increased risk of sudden death (adjusted hazard ratio 0.38, 95% confidence interval 0.05 to 2.74).In the present large cohort of patients with hypertrophic cardiomyopathy, unexplained syncope was a risk factor for sudden death. Patients with syncopal events that occurred in close temporal proximity to the initial evaluation showed a substantially higher risk of sudden death than patients without syncope. Older patients with remote syncopal events did not show an increased risk.