Pleural effusion is a common condition in critically ill patients (both clinical and surgical). Its diagnosis and classification are important for followup of patients with cardiorespiratory difficulty. Lung ultrasound is used for this purpose, but no reports have been published on its use in Cuba with critically ill patients in intensive care units. We performed lung ultrasound on 144 such patients with cardiorespiratory illnesses, average age 54 years, predominantly men (66%; 95/144), with average APACHE II score 13.6, and 22.1% mortality risk. Patients were divided into two groups: clinical (bronchopneumonia and cardiac insufficiency) and surgical (postoperative liver and kidney transplant or vascular and cardiovascular surgery) to diagnose and classify pleural effusion according to locus (right, left and bilateral) and structural pattern (I, II A, II B, III and IV). Pleural effusions were diagnosed in 81.2% (117/144) of patients (clinical 44.4%, 52/117; surgical 55.6%, 65/117). Bilateral location was the most common (68.4%, 80/117), followed by right (23.9%, 28/117) and then left (7.7%, 9/117). Structural pattern I (anechoic appearance) was observed in 61.5% of cases (72/117); 21.4% (25/117) were II A, 12.8% (15/117) II B, 3.4% (4/117) III, and 0.9% (1/117) were IV. We found no association between pleural effusion localization and ultrasound structural pattern in clinical patients (Fisher exact test 4.2 p = 0.9). In surgical patients, however, complex ultrasound patterns (II A, II B and III) were significantly more common in bilateral forms (Fisher exact test 14.1; p = 0.009). Further studies of this type in Cuba will help provide useful data for prompt treatment and followup of these patients.