OBJECTIVE To evaluate the findings of previous studies that opening of the pleura during internal mammary artery (IMA) dissection might be an important factor in increasing the operative morbidity. METHODS A randomized control trial. METHODS A university hospital. METHODS Two hundred and eighty consecutive patients with no significant pulmonary disease. METHODS Harvesting of the IMA with (130 patients) or without (150 patients) opening the pleura. METHODS Comparison of the incidence of pleural effusion, cardiac tamponade, postoperative respiratory complications and the hospital stay. RESULTS Pleural effusion occurred more often in the patients who had opening of the pleura (20% versus 5%); however, none of the patients required tapping. Postoperative bleeding with cardiac tamponade occurred in five patients in the closed pleura group. Six patients in the open pleura group had postoperative bleeding but without tamponade. The average postoperative stay was 7 days for both groups. No significant differences were recorded in postoperative respiratory complications. CONCLUSIONS Opening of the pleura during IMA harvesting does not increase the operative morbidity. It may have other advantages and is recommended in most cases of IMA harvesting.