Objective: The aim was to develop a practical method for estimation of the volume of pleural effusion using ultrasonography in mechanically ventilated patients. Design: Prospective observational study. Setting: 20-bed general intensive care unit in the university hospital. Patients and participants: 81 patients were included after initial suspicion of pleural fluid on chest supine X-ray and pre-puncture ultrasound confirming effusion. Patients with thoracic deformities, post-lung surgery, with diaphragm pathology, haemothorax, empyema and with incomplete aspiration of pleural fluid on post-puncture ultrasound were excluded. Interventions: Patients were supine with mild trunk elevation at 15ยฐ. Probe was moved upwards in posterior axillary line, and transverse section perpendicular to the body axis was obtained with pleural separation visible at lung base. The maximal distance between parietal and visceral pleura (Sep) in end-expiration was recorded. Thoracentesis was per-formed at previous probe position and volume of pleural fluid (V) recorded. Measurements and results: 92 effusions were evaluated and drained; 11 (12%) were excluded for incomplete aspiration. Success rate of obtaining fluid under ultrasound guidance was 100%; the incidence of pneumothorax or bleeding was zero. Mean Sep was 35 ยฑ 13 mm. Mean V was 658 ยฑ 320 ml. Significant positive correlation between both Sep and V was found: r = 0.72; r 2 = 0.52; p < 0.001. The amount of pleural fluid volume can be estimated with the simplified formula: V (ml) = 20 ร Sep (mm). Mean prediction error of V using Sep was 158.4 ยฑ 160.6 ml. Conclusions: Easy quantification of pleural fluid may help to decide about performing thoracentesis in high-risk patients, although thoracentesis under ultrasound guidance appears to be a safe procedure.