BACKGROUND: Pericarditis may be a serious complication of malignancy. Its significance as a first symptom of occult cancer and as a prognostic factor for cancer survival is unknown. METHODS: Using Danish medical databases, we conducted a nationwide cohort study of all patients with a first-time diagnosis of pericarditis during 1994 to 2013. We excluded patients with previous cancer and followed up the remaining patients for subsequent cancer diagnosis until November 30, 2013. We calculated risks and standardized incidence ratios of cancer for patients with pericarditis compared with the general population. We assessed whether pericarditis predicts cancer survival by the Kaplan-Meier method and Cox regression using a matched comparison cohort of cancer patients without pericarditis. RESULTS: Among 13 759 patients with acute pericarditis, 1550 subsequently were diagnosed with cancer during follow-up. The overall cancer standardized incidence ratio was 1.5 (95% confidence interval [CI], 1.4-1.5), driven predominantly by increased rates of lung, kidney, and bladder cancer, lymphoma, leukemia, and unspecified metastatic cancer. The <3-month cancer risk among patients with pericarditis was 2.7%, and the standardized incidence ratio was 12.4 (95% CI, 11.2-13.7). The 3- to <12-month standardized incidence ratio of cancer was 1.5 (95% CI, 1.2-1.7), subsequently decreasing to 1.1 (95% CI, 1.0-1.2). Three-month survival after the cancer diagnosis was 80% and 86% among those with and without pericarditis, respectively, and the hazard ratio was 1.5 (95% CI, 1.3-1.8). One-year survival was 65% and 70%, respectively, corresponding to a 3- to <12-month hazard ratio of 1.3 (95% CI, 1.1-1.5). CONCLUSIONS: Pericarditis may be a marker of occult cancer and augurs increased mortality after a cancer diagnosis.