Thoracoscopy was performed under local anesthesia in 419 patients suffering from a diffuse lung disease. In 85% of the cases diagnosis was clarified by thoracoscopy. All other cases were confirmed by means of an open lung biopsy. The best results were obtained in sarcoidosis of the stages II and III, the sensitivity being 0.98. Tumour-conditioned diffuse lung diseases were clarified in 88% of the cases; proof of an interstitial pulmonary fibrosis or interstitial pneumonia was established in 85% of the patients. Results regarding histiocytosis X were poor: thoracoscopic-bioptic proof was successful in only 42% of the patients. In 419 examinations we only detected a severe complication (air embolism). Drainage times were on the average between 4 and 5 days. On the whole, the method was characterised in the field of diagnosis of diffuse lung diseases by a high degree of sensitivity and satisfactory specificity. Both in respect of the invasiveness of the examination and its sensitivity it occupies an intermediate position between peripheral bronchoscopically obtained biopsy and surgical open lung biopsy, representing a valuable extension of the diagnostic instrumentarium if the indication is carefully considered.