Acute renal failure is a frequent complication of critical illness and optimal preventive therapy remains elusive. There is increasing evidence from animal models and some human studies that adenosine receptor antagonism by aminophylline may reduce the severity of renal impairment caused by a variety of aetiologies. We studied the renal effects of intravenous aminophylline in an unblinded, within-patient study of 20 patients admitted to a general intensive care unit following major surgery. We demonstrated that there were no adverse cardiovascular complications related to aminophylline therapy. Renal sodium and osmolar clearance increased with a non-significant trend towards increased diuresis during treatment. Creatinine clearance, however, was unchanged but the study was not designed and did not have the power to test whether aminophylline increased renal blood flow or glomerular filtration rate. We suggest the renal actions of aminophylline in critical illness merit further investigation.