The impact on drug therapy and costs of a program to identify and correct unadjusted dosage in renally impaired patients is described. The program was instituted in May 1988 by the clinical pharmacy staff at a 272-bed hospital. Each day the clinical pharmacist uses laboratory data to list patients with serum creatinine concentrations greater than 1.5 mg/dL. The pharmacist screens the pharmacy profiles of listed patients and calculates creatinine clearance for patients receiving renally eliminated drugs. If, after reviewing the patient's medical record, the pharmacist judges that a dosage adjustment may be appropriate, he writes a confidential note to the physician. From May 1988 through June 1989, 2341 patients with elevated serum creatinine were monitored. During that period, 162 notes were left; recommendations from 142 (88%) of the notes were accepted by physicians. Most of the notes were written for patients receiving antimicrobials or histamine H2-receptor antagonists. The program, which requires 20-30 minutes of pharmacist time per day, avoided $5003 in drug acquisition costs and cost $2700 to administer during the one-year period. When the costs associated with drug preparation and administration are considered, net cost avoidance was $5040. An intervention program in which notes to physicians are written when patients with abnormal serum creatinine values are receiving drugs for which a dosage adjustment appears indicated (1) has medical staff acceptance, (2) helps to satisfy standards of the Joint Commission on Accreditation of Healthcare Organizations, and (3) saves money.