Significance of the project For this large trial, with a simple design, our hypothesis is that sevoflurane will not produce greater increases in creatinine than isoflurane in patients with preexisting renal impairment. For this project an unequivocal finding would be that the 95% confidence intervals for the differences between the isoflurane and sevoflurane groups did not include clinically important differences and that any differences that were found had a p value of greater than 0.05. Unequivocal findings would lead us to two conclusions: (1) that it is highly unlikely that sevoflurane has clinically important adverse renal effects, and (2) that sevoflurane is safe to use with low fresh gas flows in patients with preexisting renal impairment.