A randomized, placebo-controlled, double-blind and sequentially analysed clinical trial to determine the efficacy of intra-operative parenteral gentamicin and vancomycin (with streptomycin in the irrigating solution) in preventing infection at the operative site following neurosurgical procedures is described. Patients receiving prophylaxis had a significantly (P = 0.046) lower operative site infection rate (2/71 = 2.8%) than those receiving placebo (9/77 = 11.7%). This difference was most apparent during an epidemic, the source of which was not evident. Moreover, a total of 13 infections (two operative site, five pneumonia and six urinary tract) occurred among 12 patients receiving prophylaxis, whereas there was a total of 31 infections (nine operative site, nine pneumonia, 10 urinary tract and three septicaemia) among 24 patients receiving placebo. A smaller quantity of antimicrobial drugs was administered postoperatively to patients receiving prophylaxis (3.96 'antibiotic-days' per patient) than to those receiving placebo (6.87 'antibiotic-days' per patient).