Abstract In 9/2022 we admitted a 69–year–old man with history of hypertension, in therapy with bisoprolol and edoxaban after recent diagnosis of persistent atrial fibrillation, with a minor pericardial effusion at the echocardiogram. He had dyspnea, fever, and atrial fibrillation with elevated cardiac frequency. The echocardiogram showed increased pericardial effusion, circumferential, about 2 cm, with important fibrinous organization, without signs of cardiac tamponade (fig1). Hemocultures and urocultures were repeatedly negative, with significant increase in flogosis parameters. Therapy with high–dose acetylsalicylic acid was started first and then Ibruprofene + colchicine with poor clinical response and persistence of symptoms and effusion. The presence of elevated fever, arthralgias, transient skin rush to the trunk and flogosis indices (CRP 17 mg/dl, ESR 97mm, WBC 11000/ul with 80% neutrophil, ferritin 2200 ng/ml), with negativity of antinucleus antibodies and rheumatoid factor, allowed to place diagnosis of adult Still’s disease (AOSD). High–dose steroid therapy (prednisone 1mg/kg) was initiated with fever regression, progressive normalization of inflammatory indices and spontaneous restoration of sinus rhythm. The 7–day echocardiogram showed almost complete resolution of pericardial fibrinous effusion (fig 2). At 2 months patient remained asymptomatic, no more pericardial effusion, no signs of flogosis. Differential diagnosis of pericarditis is often complex, and therapeutic attitude and prognosis are variable according to etiology. AOSD has a prevalence of about 1/100,000, affects mainly young adults, very late onset cases are rare. Transient, small, salmon–pink, not itchy rashes, usually concurrent with fever, can drive diagnosis, along with major and minor criteria of Yamaguchi. Often patients have serositic involvement with pleuritis or pericarditis, with tamponade needing drainage. Such forms respond well only to steroid treatment. However, cases of fibrinous pericarditis as marked as the one described have not been described; the so important fibrinous component would made an eventual pericardiocentesis difficult. Although infrequent, it is a differential diagnosis that the cardiologist must consider in cases of refractory pericarditis and pericardial effusion.