Thoroscopy and pleuroscopy are utilized to confirm the diagnosis and obtain samples for pathological analysis and tumor staging. Special stains and electron microscopy may be needed for the diagnosis. Fluoroscopically guided biopsy is diagnostic in 98% of cases. Pleural fluid is often exudative and hemorrhagic although it is not usually diagnostic except in occasional cases where malignant mesothelial cells are found.

Imaging studies included chest radiographs, CT, MRI, and PET scans. Chest films may show obliteration of the involved diaphragm and a ground glass appearance, pleural thickening, pleural encasement, and decreased size of the involved hemithorax.

TMN classification of the International Mesothelioma Interest Group stages mesothelioma as follows:
Stage I – Completely contained without adenopathy
Stage II – Intrapleural adenopathy
Stage III – Local extension into the chest wall
Stage IV – Distant metastases