The appropriateness of metronidazole use at a teaching hospital was determined by a retrospective audit. All orders for metronidazole received by the hospital pharmacy from December 26, 1986, to May 15, 1987, were identified, and the charts of the patients involved were reviewed to determine whether the drug's use was appropriate or inappropriate according to pre-established guidelines for indication, dose, dosage interval, and duration of therapy or prophylaxis. The oral route of administration was considered appropriate for patients who were taking other oral medications or who were on a diet of at least clear fluids. The acquisition cost of metronidazole from January to June 1987 was used to determine the potential cost avoidance that would result if (1) all types of inappropriate use were corrected, or (2) only the inappropriate route of administration were corrected. A total of 104 courses of therapy in 98 patients were evaluated, of which 76 were therapeutic and 28 were prophylactic. Of the 104 courses, 62% were found to be inappropriate by at least one of the criteria for appropriate use, excluding route of administration. Of the 893 doses of metronidazole administered by the i.v. route, 424 (47%) could have been given orally. A drug cost analysis revealed that $3954 (48% of the total hospital expenditure for metronidazole) could have been saved by substituting oral metronidazole for i.v. metronidazole when possible. If all criteria for appropriate use of metronidazole had been followed, $5977 could have been saved, which represents 71% of the current hospital expenditure for metronidazole.(ABSTRACT TRUNCATED AT 250 WORDS)