Adult onset Still's disease (AOSD) is characterized by spiking fevers, arthritis, rash, and involvement of multiple organs, and can be classified as self-limited, intermittent, and chronic disease groups. Cardiac manifestations include pericarditis and myocarditis. The case of this disease flared only with pericardial effusion is not reported. We describe a patient with adult onset Still's disease who was flared with pericardial effusion without other AOSD-associated symptoms, and propose that pericardial effusion should be included as a feature of flare in the intermittent disease group of adult onset Still's disease.