This retrospective study of office and hospital electronic medical records from June 1991 to June 2007 examines the occurrence of severe blood pressure (BP) elevation (>180/110 mm Hg) and the subsequent risk of cardiovascular events in a diverse set of primary care practices. A total of 18,747 patients were categorized according to BP using 3 methodologies based on the highest historical value, the first recorded value, and time-averaged antecedent values. During the follow-up period (median 3.8 years) there were 949 cardiovascular events and 80 cardiovascular-related deaths. Severe BP elevation occurred in 1566 (8.4%) patients. The age-adjusted incidence of cardiovascular events per 1000 patient-years was 5.9 in the normal BP group, 10.1 in the mild group, 15.1 in the moderate group, and 25.0 in the severe group. An episode of severe BP elevation is common in primary care practice and is associated with substantial excess cardiovascular morbidity.