Objective: We estimated the prevalence and incidence of left ventricular hypertrophy (LVH) in this large prospective cohort study of almost 20,000 participants and identified risk factors in them. Predictive factors of its appearance were evaluated along with morbidity and mortality calculations. Material and methods: LVH was defined as Minnesota Code 310 on ECG. Everyone with this code at first visit was defined as a prevalence case and those who developed it between subsequent visits were incidence cases. Risk factors at the time of the diagnosis of LVH were determined with logistic regression. Predictive factors for acquiring this ECG abnormality were determined by Poisson regression. The comparison cohort were all other participants in the Reykjavík Study stages I-V. Results: Two hundred ninety-seven men and 49 women were found to have LVH or 3.2% and 0.5%, respectively. The incidence was 25/1000/year among men and 6/1000/ year among women. Prevalence in both genders increased with increasing age. Risk factors at the time of diagnosis were systolic blood pressure (odds ratio pr. mmHg (OR) 1.02; 95% confidence interval (CI): 1.01-1.03), age (OR pr. year: 1.04; 95% CI: 1.02-1.05), silent myocardial infarction (MI) (OR: 3.18; 95% CI: 1.39-7.27) and ST-T changes (OR: 3.06; 95% CI: 2.14-4.38) among men and systolic blood pressure and age for women with similar odds ratio. Predictive factors for acquiring LVH were systolic blood pressure (incidence ratio (IR): 1.01; 95% CI: 1.01-1.02) and angina with ECG changes (IR: 2.33; 95% CI: 1.08-5.02) among men and systolic blood pressure among women (IR: 1.03; 95% CI: 1.01-1.04). In men severe smoking seemed to have a protective effect against developing LVH (IR: 0.36; 95% CI: 0.18-0.71). The risk for coronary mortality was significantly increased among women with hypertrophy (hazard ratio (HR): 3.07; 95% CI: 1.5-6.31) and their total survival was poorer with increasing time from diagnosis of LVH (HR: 2.17; 95% CI: 1.36-3.48). Conclusions: We conclude that the presence of LVH and its appearance is associated with age and increased blood pressure among both genders. Women with LVH have poorer survival than other women and they are at threefold risk of dying of ischemic heart disease. This could indicate that criteria for detecting LVH on ECG detect both mild and severe hypertrophy among men but only the severe hypertrophy cases among women. More sensitive ECG methods may have to be used to detect mild, moderate and severe LVH among both genders in order to differentiate the severity of LVH based on the ECG diagnosis.