In a retrospective study the results of medical thoracoscopy in 147 patients were reviewed; 136 of the patients had pleural effusion and 11 patients had diffuse pulmonary infiltration. All the pleural exudates were initially screened three times successively and found to be sterile and without tumour cells. All thoracoscopies were performed with local anaesthesia, with the 'open technique', and nine different doctors performed the thoracoscopies. The overall diagnostic sensitivity was 90.4%. The results demonstrated 62% with malignancy of the pleura, and 38% revealed benign pleural diseases, among them 2% with tuberculosis. The sensitivity for malignancy was found to be 88% and the specificity 96%. The most common primary lung cancer with involvement of the pleura was the adenocarcinoma (62%), and the most common metastatic tumour originated from the breast (28%). The sensitivity for tuberculosis was 100% and the specificity 100%. No mortality was found, and the morbidity was low at about 0.6% (empyema, pleuro-cutaneous fistula, transcutaneous growth of tumour (mesothelioma)). In 64% of the patients the thoracoscopy resulted in treatment (pleurodesis, antituberculous treatment, chemotherapy and peroral steroid therapy). The medical diagnostic thoracoscopy in local anaesthesia is a simple, low-cost investigation with a relatively high diagnostic accuracy, no mortality and a low morbidity.