One hundred thirty patients undergoing major thoracotomy between June 1991 and June 1992 at The Royal Brompton Hospital, London, were analyzed; renal impairment developed in 31 patients (24%). The mortality and morbidity was significantly greater for the renal impairment group. Six patients (19%) with renal impairment died after operation, in contrast to 0 of the 99 patients in whom renal impairment did not develop. The average length of hospital stay for the patients with renal impairment was 12 days compared with 8 days for the normal renal function group (p << 0.001). Five factors were highly significantly associated with renal impairment: a past history of renal impairment or diuretic intake, undergoing pneumonectomy, postoperative infection, and blood loss (p < 0.001). The most important of these appears to be postoperative infection or blood loss, as they also were associated with death (p = 0.01). Other factors less significantly associated with renal impairment included a past history of hypertension, ischemic heart disease, intraoperative gentamicin, and epidural analgesia (p < 0.01). This study emphasizes that thoracotomy must be considered carefully in patients with these predisposing factors, particularly if pneumonectomy is likely. Care must be taken in the use of aminoglycosides and epidural analgesia. Maintenance of renal blood now by careful control of hemodynamic indices appears to be the most important intervention.