Pericarditis may signal the presence of cancer, even in the absence of other clinical or paraclinical signs. Corollary, the following question arises: Could the discovery of a newly developed pericarditis be used in patients with known neoplasia as a marker of cancer progression? In an attempt to find an answer to this question, this two-centre study included 341 consecutive patients with a confirmed diagnosis of cancer and evidence of pericardial effusion at echocardiography and/or CT/MRI scan. The patients’ data were collected retrospectively if they further fulfilled the following inclusion criteria: available medical data from confirmation of pericarditis until evidence of cancer progression or until at least 12 months without progression. The average age of the patients was 62.16 years (22–86 years), and the study comprised 44.28% males and 55.71% females. All types of the most common neoplasms were represented. The results showed that 85.33% of patients had cancer progression temporally linked to pericarditis. Of these, 41.64% had cancer progression within 18 months after the diagnosis of pericarditis with a median time to progression of 5.03 months, ranging from 0 to 17 months; 43.69% had progression within a maximum of 2 months before the diagnosis of pericarditis. Only 14.66% had no cancer progression during the observation period. We concluded that pericarditis could be a sensitive marker of cancer evolution that could be widely used as a follow-up investigation for cancer patients as a marker of progression or recidive.