A clear understanding of the significance of wound contamination as well as knowledge of the microbial flora that could be expected are needed in order to administer a rational and effective antibiotic treatment for open fractures. We have conducted a prospective study of the contaminating microbial flora in 89 open fractures. In addition, two more cultures were taken of all wounds not primarily closed. Wound and fracture healing were followed in all patients. Most fractures were Gustilo grade II (58.4%) caused by work-related accidents. Wound cultures were positive in 83% of all fractures, and a total of 84 strains of bacteria were isolated. In 39.3% of cultures, various species of aerobic Gram-negative rods (most commonly Pseudomaonas aeruginosa) were retrieved, followed by Staphylococcus epidermidis (34.5%) and Staphylococcus aureus (26.1%). Repeat cultures were mostly either negative (59.5%) or grew saprophytic organisms that were usually nonpathogenic (such as various species of saprophytic Bacillus bacteria). The only cases that developed deep wound infection were those where a repeat culture 1 day after debridement grew the same organisms as the initial organisms. We conclude that (a) most open fractures are already contaminated upon the patient's arrival at the emergency department, in many cases by potentially pathogenic staphylococci and Gram-negative organisms; (b) contaminating organisms are community acquired and, as such, are sensitive to most routine antibiotics; and (c) persistence of the same organisms in a repeat culture taken 1 day after debridement signifies technical failure of debridement and a subsequent very high risk of infection. Therefore, achieving adequate wound asepsis immediately following debridement is of the utmost importance.