Adult onset Still's disease is a rare systemic inflammatory disease of unknown origin. It is common to involve liver and spleen, and less often lungs. A 24-year-old man presenting with spiking fever, pulmonary infiltrations, and pleural effusion on the right side mimicking pneumonia. All serologic tests of infectious causes, antinuclear antibody, and rheumatoid factor were negative. He was diagnosed as Adult onset Still's disease according to the criteria of Yamaguchi. Adult onset Still's disease is considering in the differential diagnosis pneumonia that is unresponsive to antimicrobial treatment, negative cultures and serologically diagnostic laboratory tests, and high-level serum ferritin.