A single prophylactic dose of vancomycin was given in 143 supratentorial craniotomies with a bone flap and 26 suboccipital craniotomies. No antimicrobial agents were given to two similar groups of patients: 172 with supratentorial craniotomy and 19 with suboccipital craniotomy. The infection rate in the vancomycin groups was significantly lower (p less than 0.013), and bone-flap infections alone, the most common infection after supratentorial craniotomy, were significantly fewer (p less than 0.042). Antimicrobial prophylaxis is recommended only for selected high-risk groups. Since a bone flap is devascularized, its resistance to infection is reduced and, once infected, it usually requires surgical removal. Patients undergoing craniotomy with a bone flap therefore form a high-risk group, and antimicrobial prophylaxis is justified.