BACKGROUND: Acute pericarditis occasionally requires invasive treatment, and may recur after discharge. However, there are no studies on acute pericarditis in Japan, and its clinical characteristics and prognosis are unknown. METHODS: This was a single-center, retrospective cohort study of clinical characteristics, invasive procedures, mortality, and recurrence in patients with acute pericarditis hospitalized from 2010 to 2022. The primary in-hospital outcome was adverse events (AEs), a composite of all-cause mortality and cardiac tamponade. The primary outcome in the long-term analysis was hospitalization for recurrent pericarditis. RESULTS: The median age of all 65 patients was 65.0 years [interquartile range (IQR), 48.0-76.0 years], and 49 (75.3 %) were male. The etiology of acute pericarditis was idiopathic in 55 patients (84.6 %), collagenous in 5 (7.6 %), bacterial in 1 (1.5 %), malignant in 3 (4.6 %), and related to previous open-heart surgery in 1 (1.5 %). Of the 8 patients (12.3 %) with in-hospital AE, 1 (1.5 %) died during hospitalization and 7 (10.8 %) developed cardiac tamponade. Patients with AE were less likely to have chest pain (p = 0.011) but were more likely to have symptoms lasting 72 h after treatment (p = 0.006), heart failure (p < 0.001), and higher levels of C-reactive protein (p = 0.040) and B-type natriuretic peptide (p = 0.032). All patients complicated with cardiac tamponade were treated with pericardial drainage or pericardiotomy. We analyzed 57 patients for recurrent pericarditis after excluding 8 patients: 1 with in-hospital death, 3 with malignant pericarditis, 1 with bacterial pericarditis, and 3 lost to follow-up. During a median follow-up of 2.5 years (IQR 1.3-3.0 years), 6 patients (10.5 %) had recurrences requiring hospitalization. The recurrence rate of pericarditis was not associated with colchicine treatment or aspirin dose or titration. CONCLUSIONS: In acute pericarditis requiring hospitalization, in-hospital AE and recurrence were each observed in >10 % of patients. Further large studies on treatment are warranted.