Mucormycosis is a rare complication in immunocompromised patients. Antemortem diagnosis of mucormycosis is difficult and often incorrect. We report a case of pulmonary mucormycosis caused by Cunninghamella bertholletiae in an elderly man with interstitial pneumonia. The diagnosis of mucormycosis was established by bronchoalveolar lavage. A coexisting immune deficiency condition was considered. Lung cancer was suspected because of an elevated progastrin-releasing peptide level and bilateral hilar and mediastinal lymphadenopathy; it was diagnosed after performing endoscopic ultrasound-guided fine-needle aspiration. Treatment by intravenous liposomal amphotericin B was effective, but relapse occurred because of bone marrow suppression caused by chemotherapy for lung cancer. Treatment for mucormycosis was resumed, but the patient died of carcinomatous lymphangiosis. Autopsy confirmed the diagnosis of pulmonary mucormycosis and revealed refractory anaemia with small cell lung cancer. Mucormycosis often occurs in immunocompromised patients, but this case is rare because the mucormycosis was diagnosed before the diagnosis of malignancy. Because prognosis is often poor, the possibility of coexisting malignancies should always be investigated in patients with mucormycosis infections.