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BACKGROUND Approximately a quarter of patients with hypertrophic cardiomyopathy (HCM) have left ventricular outflow tract obstruction (LVOTO) and this has been associated with poor outcome. The association between LVOTO and survival in relation to other risk factors for sudden cardiac death (SCD) has not, however, previously been studied. OBJECTIVE To determine whether the relationship between LVOTO and SCD is influenced by the presence of other risk factors. DESIGN AND INTERVENTION This observational study took place at St George's Hospital, London, UK between January 1988 and March 2002 and enrolled patients aged 16 years or older with a diagnosis of HCM. Patients were excluded if they had other conditions known to cause ventricular hypertrophy. Patient assessment included physical examination, 48h ambulatory electrocardiography, and exercise on either a bicycle or treadmill with concurrent analysis of systolic blood pressure and respiratory gases. Patients also underwent echocardiography for the measurement of left ventricular (LV) outflow gradient. OUTCOME MEASURES The outcomes were SCD, internal cardioverter-defibrillator (ICD) discharge, heart transplantation, death from congestive heart failure, other cardiovascular death (e.g. stroke, thromboembolism, or myocardial infarction), or noncardiac death. RESULTS A total of 917 eligible patients were enrolled in the study; 60.4% were male and the mean age was 43 years (range 16-88 years). Patients were divided into five groups on the basis of increasing LV outflow gradient. LVOTO was defined as an LV outflow gradient of 30mmHg or higher and was present in 31.4% of patients. After a median follow-up of 61 months, 122 patients had died, undergone a heart transplant, or experienced an ICD discharge; 40.2% of these individuals had LVOTO. The 5-year survival rate for allcause death or transplantation was significantly lower in patients with an obstruction than in those without (86.5% versus 90.1%; P=0.006) and there was an incremental increase in mortality and incidence of transplantation with increasing severity of LVOTO (relative risk [RR] per 20mmHg increase = 1.24; P = 0.003). Similarly, the 5-year survival rate for SCD or ICD discharge was lower in patients with LVOTO than in those without obstruction (91.4% versus 95.7%; P=0.004). There was also a trend towards worse survival with increasing LV outflow gradient (RR per 20mmHg increase=1.36; P=0.001). In patients with LVOTO, there was a significant correlation between 5-year all-cause survival and NYHA status (91.0% for NYHA class I versus 82.6% for NYHA class Ill/IV; P=0.002). Five-year survival was not significantly affected by a history of chest pain or syncope. Multivariate analysis indicated that LVOTO was an independent predictor of SCD and ICD discharge; the RR for an LV outflow gradient of 90 mmHg or higher, when compared with patients with less severe or no LVOTO, was 3.8 (P= 0.005). CONCLUSION The risk of SCD or ICD discharge is higher in patients with LVOTO, and the risk increases incrementally with severity of obstruction. [ABSTRACT FROM AUTHOR]