The value of the A-mode ultrasonic technique and the radiologic method in the diagnosis of pleural effusion was assessed in 116 patients with diseases of the pleura. Ultrasonic and radiologic examinations, as well as needle punctures, were performed, and the results were compared statistically. The pleural fluid was detected by ultrasound in 93 percent (74) and by radiologic examination in 83 percent (66) of the 80 cases with such fluid. The absence of fluid was established by ultrasound in 89 percent (32/36) and by radiologic examination in 61 percent (22/36). For the first time the superiority of the ultrasonic method over the radiologic one was demonstrated, and the difference was most obvious in cases of small pleural effusion. Ultrasound permitted the detection of very small amounts (even 3 to 5 ml) of loculated pleural fluid. In contrast to the radiologic method, ultrasound permitted easy differentiation between loculated pleural fluid and pleural thickenings. The ultrasonic method appeared especially useful in the accurate localization and precise indicating of the site for needle aspiration of even the smallest fluid collections. It made possible thoracocentesis in 94 percent (154) of 163 instances. The practical value of the ultrasonic method, both in establishing diagnosis and in treatment, is emphasized.