NEUROSURGICAL OPERATIONS HAVE traditionally been classified along the lines of general surgical procedures. A prospective study, during an 18-month period, was undertaken in 2249 patients undergoing neurosurgical procedures to establish and evaluate a method of classifying surgical cases by the use of specific neurosurgical criteria. Patients were placed in one of five categories according to the level and type of contamination at the time of surgery. Infection included all abnormal wounds and was documented as deep when infection occurred beneath the galea (subgaleal pus, osteitis, abscess/empyema, ventriculomeningitis) and as superficial if only the scalp (including wound erythema) was involved. A statistically significant difference in the sepsis rate was found in the different categories (P< 0.0001). Of the 342 “dirty cases,” 9.1% of patients developed further wound sepsis. Concomitant cerebrospinal fluid fistulae (44%), second operations (11.8%), and patients with penetrating injuries (9.2%) were the major factors implicated in sepsis in the “contaminated” category (9.7%). In the “clean contaminated” category, a sepsis rate of 6.8% was found. Prolonged surgery (longer than 4 hours) was also implicated in higher infection rates (13.4%). This study strongly supports the separation of patients who have foreign materials implanted (sepsis rate = 6.0%) from “clean” patients, essentially cases categorized as having no known risk factors that may affect sepsis, in whom a sepsis rate of 0.8% was found (P< 0.001). Importantly, surgery for the repair of so-called “clean” neural tube defects in neonates requires separate consideration. An infection rate of 14.8% existed in this subgroup. A uniform system of reporting wound abnormalities is also proposed.