Vacuum-assisted closure has become a new technique in the challenging management of contaminated, acute, and chronic wounds. Although promising clinical results have been described, scientific proof to substantiate the mechanism of action of this therapy is scarce. In the present study, we examined whether the positive effect on wound healing found in vacuum-assisted closure–treated wounds could be explained by an effect on the bacterial load. Fifty-four patients who needed open wound management before surgical closure were included in this study. Wounds were randomized to either vacuum-assisted closure therapy (n= 29) or treatment by conventional moist gauze therapy (n= 25). Healing was characterized by development of a clean granulating wound bed (“ready for surgical therapy”) and reduction of wound surface area. To quantify bacterial load, biopsies were collected. No significant difference was found in time needed to reach “ready for surgical therapy” comparing both therapies. Wound surface area reduction was significantly larger in vacuum-assisted closure–treated wounds: 3.8 ± 0.5 percent/day (mean ± SEM) compared to conventional-treated wounds (1.7 ± 0.6 percent/day; p < 0.05). The total quantitative bacterial load was generally stable in both therapies. However, nonfermentative gram negative bacilli showed a significant decrease in vacuum-assisted closure–treated wounds (p < 0.05), whereas Staphylococcus aureus showed a significant increase in vacuum-assisted closure–treated wounds (p < 0.05). In conclusion, this study shows a positive effect of vacuum-assisted closure therapy on wound healing, expressed as a significant reduction of wound surface area. However, this could not be explained by a significant quantitative reduction of the bacterial load.