We describe the clinical and pathologic findings of a 68-year-old smoker with previous asbestos exposure who presented with spontaneous hydropneumothorax and was diagnosed with synchronous undifferentiated lung carcinoma and incidental malignant pleural mesothelioma. The synchronous occurrence of these two neoplasms is an extremely rare event with fewer than 20 reported cases in the English literature. The accurate diagnosis of synchronous tumors can be extremely challenging and the identification of a concomitant mesothelioma in our case was not made until an extensive immunohistochemical analysis was done on the resection specimen. Spontaneous pneumothorax occurs much more commonly in patients with malignant mesothelioma than with primary lung carcinomas. Consequently, although synchronous pleural mesotheliomas and lung carcinomas are infrequent, this diagnosis should be considered when a patient with a lung mass and a history of asbestos exposure presents with spontaneous pneumothorax and pleural thickening on imaging. Identification of synchronous tumors is of critical importance for determining the patient’s stage and management and can have significant medicolegal implications should the patient seek compensation.