Bronchopulmonary cancer is the leading cause of cancer deaths globally. Rheumatoid arthritis is one of the risk factors for lung cancer, and those who use methotrexate have a higher risk of developing lung cancer. We present the case of an 80-year-old patient who is a former smoker and is known to have rheumatoid arthritis, being treated using methotrexate; they were brought by ambulance to the emergency room for coughing with ineffective expectoration, dyspnea on slight exertion, and right-lateral chest pain with onset about one month prior and progressive worsening. Imaging showed a 7 cm/6 cm LID tumorous lung formation with parietal invasion and C7 rib lysis, as well as diffuse fibrotic interstitial changes predominantly in the lower lobes. An ultrasound-guided transthoracic lung biopsy was performed, and histopathological examination established the diagnosis of invasive squamous cell lung carcinoma, G2. In conclusion, the chest pain interpreted by the patient as rheumatic pain delayed the diagnosis of lung cancer; the patient presented rather late to the hospital once respiratory failure set in.