The sensitivity of ultrasonographic diagnosis of pleural fluid accumulations and the value of ultrasonographically guided thoracentesis were studied prospectively. One hundred ten patients were examined after abdominal operations and chest trauma. Most of the examinations were performed in a half-sitting position. Pleural fluid of clinical relevance, diagnosed by real-time ultrasonography, was treated by thoracentesis under ultrasonographic guidance in 38 cases. The amount of aspirated fluid ranged from 150 to 1350 ml. The sensitivity of the method was 97.1%. The complication rate was 2.6%. There are now 240 patients treated by ultrasonographically guided thoracentesis and 160 cases with pleural drainage. In our view, ultrasonographically guided thoracentesis represents the method of choice in critically ill and immobile patients.