Wound contamination with endogenous bacterial scalp flora plays an important role in the pathogenesis of postoperative neurosurgical infections. To assess the effect of preoperative antiseptic shampoos on the emergence of resident scalp flora during surgery and subsequent wound contamination, we randomized 151 neurosurgical procedures into four study groups: group A—preoperative shampoos with chlorhexidine, surgical scalp preparation with chlorhexidine; group B-no shampoos, surgical preparation with chlorhexidine; group C-shampoos with iodophor, surgical preparation with iodophor; group D-no shampoos, surgical preparation with iodophor. Quantitative cultures of the scalp were obtained preoperatively and at the end of surgery, and qualitative wound cultures were taken prior to wound closure. Group A had the lowest concentration of bacteria on the scalp both preoperatively and postoperatively (median range = 30 [0-5.7 × 105] and 0 [0-2.5 x 103] respectively). Group A also had significantly fewer positive postoperative scalp cultures (29%) than groups B (51%), C (58%), and D (53%) (P<0.05), as well as fewer positive wound cultures (20% v 25%, 42%, and 30% respectively). A density of bacteria on the scalp of > 102/4cm2 best predicted the presence of bacteria in the wound. Repeated preoperative shampoos with chlorhexidine reduce intraoperative emergence of resident skin flora and subsequent contamination of the wound.