The efficacy and toxicity of once-daily (od) versus twice-daily (bd) dosing of vancomycin was compared in 121 hospitalized patients. Eighteen patients were then withdrawn from the study. Clinical and bacteriological responses were evaluated in all patients (n = 103). Nephrotoxicity was assessed in patients who did not receive nephrotoxic agents (n = 76). Ototoxicity was assessed in patients who completed two audiograms and were not receiving ototoxic agents (n = 63). No significant difference was found between the two groups for favourable clinical response: 47/51 (92.1%) and 49/52 (94.2%) in the od and bd groups, respectively. In 34 patients vancomycin was the only effective antibiotic. Fifteen of 18 (83.3%) evaluated episodes in the od and 12/16 (75.0%) evaluated episodes in the bd group showed a favourable bacteriological response. There were no significant differences between the od and bd groups for all adverse events. Nephrotoxicity developed in 4/37 (10.8%) and 3/39 (7.7%) patients, respectively. Hearing loss developed in 1/31 (3.2%) and 5/32 (15.6%). Phlebitis occurred in 7/51 (13.7%) and 12/52 (23.0%). Red man syndrome occurred in 7/51 (13.7%) and 5/52 (9.6%) in od and bd groups, respectively. The efficacy and safety profile of od administration of vancomycin is similar to that of the customary, but less convenient, bd administration.