A retrospective survey of 850 spinal procedures was conducted to determine risk factors for postoperative spinal infection. Some 574 posterior and 276 anterior procedures were evaluated. Infections were classed as early (within 19 weeks postoperatively) and late (> or = 20 weeks postoperatively), as well as deep and superficial infections. Previously postulated risk factors (i.e., diabetes, corticosteroid therapy, previous spinal surgery, obesity, chronic infection, and smoking) were investigated. Furthermore, it was hypothesized that three additional factors: extended preoperative hospitalization, prolonged operation duration, and high blood loss were risk factors not previously considered. A postoperative infection was evident in 22 of 850 procedures (19 early/three late infections, four superficial/18 deep infections). Most infections (n = 21) occurred with posterior instrumentation. Previously described factors were proven as risk factors. Extended preoperative hospitalization (p < 0.04), high blood loss (p < 0.01), and prolonged operation time (p < 0.001) were additionally determined as risk factors. The results suggest that routine prophylactic antibiotics might be unnecessary in anterior surgery, whereas extended prophylactics should be considered in posterior instrumentation, especially when risk factors are apparent.