BACKGROUND: Recently, COVID-19 infection has become a public health concern. On March 12th, 2020, the World Health Organization (WHO) announced it as a global pandemic. Early diagnosis of atypical cases of COVID-19 infection is critical in reducing the transmission and controlling the present pandemic. In the present report, we described a patient with the chief complaints of dyspnea and dry cough referred to the oncology center at Imam Khomeini Hospital, Tehran with the differential diagnosis of lung cancer who was diagnosed and treated for COVID-19 infection in follow up. CASE PRESENTATION: A 59-year-old patient complained of fever, dry cough, and dyspnea from two weeks ago. The patient had been referred to this center with the differential diagnosis of lung cancer due to the massive pleural effusion in initial chest CT scan. Dyspnea was the patient's main complaint at the time of admission in this center and the oxygen saturation was 84%. In the new chest CT scan, similar findings were observed. Due to the severe respiratory distress, a chest tube was placed in the chest cavity to remove the pleural effusion fluid on day one. The patient's felt relieved immediately after the procedure; however, the oxygen saturation did not raise above 85% despite the oxygen therapy. The cytology of pleural fluid was negative for malignant cells. On day 2, the lymphopenia and high level of CRP suggested the COVID-19 infection. Therefore, a control chest CT scan was conducted and the test for COVID-19 was performed. The CT report indicated the clear pattern of COVID-19's lung involvement in the absence of pleural effusion. Thus, the treatment for COVID-19 was immediately initiated. On day 4, the test reported positive for COVID-19. CONCLUSION: Currently, it is important to bear in mind the COVID-19 infection in evaluating the patients with respiratory symptoms. This report indicated how misleading the presentation of chest CT scan could be in clinical judgment. Therefore, we recommend ruling out the COVID-19 infection in all the patients with any pattern of lung involvement to avoid missing the potential cases of this vicious infection.