The objective of this paper is to describe clinical features, diagnostic methods and outcome in Pneumocystis jirovecii pneumonia (PCP) in three patients after cardiovascular surgery. PCP diagnosis was established by identification of typical organisms in tracheal aspirate using modified Giemsa stains. The three patients reported here had major cardiac surgery with extracorporeal circulation on consecutive days, and were all HIV negative. During the postoperative period, all patients presented lower values of oxygen saturation than during the preoperative period, even though the X-rays of the chest were normal. The diagnosis of PCP was established during ICU stay. The first patient suffered three additional surgical procedures for hemostasis, and was diagnosed with PCP in 11th postoperative day; he was treated with trimethoprim-sufamethoxazole intravenously. The second patient had an unfavorable evolution and developed fever and respiratory failure on the 2nd postoperative day. He was diagnosed with PCP on the 8th postoperative day; therapy with trimethoprim-sufamethoxazole per os was started, but the patient died on the 10th postoperative day. The third was diagnosed with PCP on the second postoperative day. He was treated with trimethoprim-sufamethoxazole for 12 days and he was discharged in stabile condition. He had no respiratory symptoms at follow-up visits.PCP should be suspected in cardiovascular surgery patients who have difficult postoperative evolution with lower values of the oxygen saturation and normal chest X-ray. In these cases, it is mandatory to do a bronchial aspirate analysis for typical organisms suggesting PCP. In typical cases we should start the prophylactic therapy with trimethoprim-sufamethoxazole intravenously.