In our pilot study of 30 patients who were on gentamicin, the adequate peak level (5-10 mg/dl) was found in 27 per cent, trough level (less than 2 mg/dl) in 67 per cent and both peak and trough level in only 10 per cent. We have several reasons to believe that it is due to inappropriate dosage prescribing. The objective of the study is to test the efficacy of using a recommended dosage schedule for rational dosage prescribing of gentamicin. All medical residents were invited to attend a 2-hour session on gentamicin pharmacokinetics. The problems and the use of recommended dosage schedule were extensively discussed. Then serum gentamicin levels were re-monitored by Fluorescence Polarization Immunoassay (TDx). Of 73 gentamicin recipients, 39 (53%, C gr.) were given at the correct dosage according to the schedule and 34 (47%, IC gr) were given incorrectly. The characteristics of the patients in both groups were not different. The appropriate peak, trough, peak and trough levels in C gr were 87, 80, 67 per cent respectively compared with 24, 80 and 9 per cent in the IC group, p less than 10(-4) for peak, peak and trough. The incidence of nephrotoxicity was not different between the two groups. The patients' outcomes were difficult to assess because most of the patients received antibiotic combinations. However, case fatality of documented gram-negative infection tended to be less in the C group. It is concluded that dosage schedule can improve rational dosage prescribing of gentamicin. Most of the patients given gentamicin may not need monitoring.(ABSTRACT TRUNCATED AT 250 WORDS)