Organizing pneumonia (OP) can mimic several parenchymal lung diseases. A 39 year old man tailor, complaining of fever, cough and dyspnea was admitted to our hospital. Examination of the chest revealed rales at the bilateral lower-middle zone. The posteroanterior chest radiograph showed alveolar opacity at the right lower zone. The patient was hospitalized with the probable diagnosis of bacterial pneumonia, and started cephtriaxon 2gr/day and dirithromycin 500 mg/day empirically. Clinical and radiological improvement didn't appear. PPD and acid fast bacilli in sputum were negative. Two weeks later, the chest radiograph showed that the opacities were migrated to the right middle zone. Open lung biopsy was done. As pathologic findings were consistent with OP, corticosteroid treatment was given. The alveolar opacities disappeared at the control HRCT two months after the start of steroid therapy. Early diagnosis is life-saver and OP must be included in the differential diagnosis when pulmonary infiltrates don't regress with treatment.