BACKGROUND Incidental durotomy during spine surgery is a common occurrence, with a reported incidence ranging from 3% to 16%. Risk factors identified by prior studies include age, type of procedure, revision surgery, ossification of the posterior longitudinal ligament, gender, osteoporosis, and arthritis. However, these studies are largely univariate analyses using retrospectively recorded data. OBJECTIVE To identify and quantify statistically significant risk factors for inadvertent durotomy during spine surgery. METHODS Multivariate analysis of prospectively collected registry data. The University of Washington Spine End Results Registry 2003 and 2004 is a compilation of prospectively collected detailed data on 1,745 patients who underwent spine surgery during 2003 to 2004. METHODS One thousand seven hundred forty-five patients underwent spine surgery from 2003 to 2004 at our two institutions. METHODS Cardiac, pulmonary, gastrointestinal, neurologic, renal, and urologic complications defined a priori data collection. METHODS Using these data, univariate and multivariate statistical analyses were performed to identify and quantify risk factors for incidental durotomy during spine surgery. Relative risk (RR) values with valid confidence intervals and p values were determined using these data. RESULTS Our multivariate analysis demonstrated that age, lumbar surgery, revision surgery, and elevated surgical invasiveness are significant risk factors for unintended durotomy. Of these, revision surgery was the strongest risk factor for dural tear (RR, 2.21). Diabetes was a significant risk factor in the univariate analysis but not in the multivariate analysis. CONCLUSIONS Revision surgery, age, lumbar surgery, degenerative disease, and elevated surgical invasiveness are significant risk factors for unintended durotomy during spine surgery. These data can be useful to surgeons and patients when considering surgical treatment.