Acetoacetate is known to spuriously raise serum creatinine concentration in patients with diabetic ketoacidosis. Its effect on urinary creatinine has not been studied. Since the renal threshold for ketoacids is low, large amounts of acetoacetate may be present in the urine of uncontrolled diabetics. We investigated this interference using three different automated analysers. We found that +3 or +4 reactions with Ketostix, equivalent to greater than 10 - 15 mmol/L of acetoacetate caused significant interference with the Abbott VP and Beckman Astra instruments. This could cause errors in the calculation of creatinine clearance especially when serum creatinine is close to a normal level. We recommend that measurement of creatinine clearance be delayed until better diabetic control is achieved or the creatinine be measured by a method which is free from ketone interference, e.g., by the Dupont aca.