Background— Constrictive pericarditis (CP) is considered a rare, dreaded possible complication of acute pericarditis. Nevertheless, there is a lack of prospective studies that have evaluated the specific risk according to different etiologies. The aim of this study is to evaluate the risk of CP after acute pericarditis in a prospective cohort study with long-term follow-up. Methods and Results— From January 2000 to December 2008, 500 consecutive cases with a first episode of acute pericarditis (age, 51±16 years; 270 men) were prospectively studied to evaluate the evolution toward CP. Etiologies were viral/idiopathic in 416 cases (83.2%), connective tissue disease/pericardial injury syndromes in 36 cases (7.2%), neoplastic pericarditis in 25 cases (5.0%), tuberculosis in 20 cases (4.0%), and purulent in 3 cases (0.6%). During a median follow-up of 72 months (range, 24 to 120 months), CP developed in 9 of 500 patients (1.8%): 2 of 416 patients with idiopathic/viral pericarditis (0.48%) versus 7 of 84 patients with a nonviral/nonidiopathic etiology (8.3%). The incidence rate of CP was 0.76 cases per 1000 person-years for idiopathic/viral pericarditis, 4.40 cases per 1000 person-years for connective tissue disease/pericardial injury syndrome, 6.33 cases per 1000 person-years for neoplastic pericarditis, 31.65 cases for 1000 person-years for tuberculous pericarditis, and 52.74 cases per 1000 person-years for purulent pericarditis. Conclusions— CP is a relatively rare complication of viral or idiopathic acute pericarditis (<0.5%) but, in contrast, is relatively frequent for specific etiologies, especially bacterial.