BACKGROUND There are conflicting reports on the effects of diltiazem treatment on renal function in surgical patients. We sought to determine whether diltiazem treatment alters renal function in patients undergoing major thoracic surgery. METHODS In a prospective study, 330 patients scheduled for elective thoracic surgery received either IV diltiazem (n = 167) or placebo (n = 163) immediately after the operation and orally thereafter for 14 days in an effort to prevent postoperative atrial arrhythmias. Serum creatinine and BUN levels were compared before and during the first postoperative week. RESULTS Patients treated with diltiazem were similar to control subjects in terms of age (mean +/-SD, 66 +/- 10 years vs 67 +/- 10 years, respectively), baseline serum creatinine or BUN levels, prevalence of comorbid conditions, and surgical characteristics. During the first 5 postoperative days, the two groups did not differ in terms of serum creatinine or BUN levels. The incidence of renal failure was 0.6% in the diltiazem group and 1.2% in the placebo group (difference was not significant). There was no difference in the length of hospitalization or mortality rate. CONCLUSIONS In patients without renal disease who are undergoing elective thoracic surgery, prophylactic diltiazem treatment did not alter postoperative renal function.