We have frequently experienced that actual serum concentrations were higher than those predicted during vancomycin therapy. In the present study, we investigated retrospectivelyto find out the characteristics of the patients whose elimination rate distinctly decreased. In elderly patients, renal dysfunctional patients and patients with low elimination rate constant, and mean prediction error induced by the difference of predicted concentrations to actual concentrations showed great values. No correlation was noted between the data of hepatic function and elimination rate constants, although mean prediction error significantly corralated with the serum albumin and cholinesterase concentrations. The patients who had to change the dose interval did not show the data of abnormal renal function. These findings suggest that the dosage requirements of vancomycin and dose interval could not be determined only by the renal function data.