Hypereosinophilia is unusual in rheumatoid arthritis (RA), but can occur in severe long-lasting disease, especially in patients with extra-articular manifestations and high titers of rheumatoid factor (RF). The association of RA and hypereosinophilic syndrome (HES) remains yet poorly known. We present a case of a 46 years old woman with long-standing untreated RA, that presented to emergency department with severe symptoms of constrictive pericarditis with cardiac tamponade and bilateral pleural effusion, that progressed to cardiac arrest, associated to symmetrical polyarthritis and pruritic erythematous skin papules. She was submitted to urgent pericardial drainage and partial pericardiotomy. Laboratory analyses revealed hypereosinophilia, and elevated inflammatory parameters and immunoglobulin E. The histological study of the pericardium showed results consistent with inflammatory fibrinous pericarditis. Taking into account the presence of some characteristics that are usually present in cases of reactive HES instead of idiopathic HES, and after an intensive diagnostic study, that could rule out other potential causes of secondary HES, the diagnosis of HES associated with RA was made. She started glucocorticoids during hospitalization and methotrexate 15mg per week at the first outpatient rheumatology visit. After 12 weeks of treatment, we considered that she was in clinical and analytical remission, consistently maintaining that after a complete tapering of glucocorticoids. This case illustrates that clinicians should be aware that HES (including severe life-threatening cases) can occur in patients with RA, especially in cases of long-lasting disease with high titters of RF and without treatment, even in the absence of extra-articular features.