Six hundred patients underwent diagnostic flexible fiberoptic bronchoscopy (FFB). The two diseases most frequently encountered were bronchogenic carcinoma in 330 patients (55 percent) and bacterial infection in 94 (16 percent). A positive cytology on biopsy material was obtained in 279 of 330 patients (85 percent) with primary lung cancer. Fluoroscopy was a valuable aid in diagnosing bronchogenic carcinoma, since 42 percent of the tumors were not visible endoscopically and required fluoroscopic control for placement of the biopsy instrument. Of the 55 patients with hemoptysis and negative chest x-ray films, nine (15 percent) had fiberoptically visible endobronchial carcinomas! In addition, two patients with carcinoma of the larynx and one with carcinoma of the nasopharynx were discovered. Transbronchial biopsy (TBB) in 68 patinets with diffuse and localized disease achieved an overall 69 percent diagnostic success, including a correct diagnosis in each of four patients with Pneumocystis carinii pneumonia. Brush biopsy provided additional valuable laboratory data in bacterial, mycobacterial and cytomegalovirsu infectious but had a poor yield in Pneumocystis infection. Complications as a result of forceps biopsy were minimal, except for brisk bleeding in six patients.