Objective: Our study analyzed the incidence of posterior pelvic injury patterns and their influence on the surgical treatment of transverse-oriented acetabular fractures. Methods: Fifty-one transverse-oriented acetabular fracture cases admitted between 1999 and 2013 were evaluated retrospectively. Comparative studies were performed for groups organized by acetabular fracture type, degree of sacroiliac separation, and postoperative reduction quality. Results: Associated posterior pelvic injuries were found in 34 (66.7%) of the 51 patients. There were 32 sacroiliac separations in the 34 patients with associated posterior pelvic injury, and ipsilateral sacroiliac separations were more frequent in this subgroup. Measurements guided by computerized tomography showed that 16 sacroiliac separations were <= 0.5 cm (mean=0.43 +/- 0.14 cm), 10 were 0.5-1 cm (mean=0.73 +/- 0.17 cm), and the remaining 6 were >1 cm (mean=1.55 +/- 0.15 cm). In the group of 34 patients with associated posterior pelvic injury, acetabular reduction was anatomic in 19 (55.9%) patients, imperfect in 10 (29.4%) patients, and poor in 5 (14.7%) patients. For isolated acetabular fractures, reduction rates were as follows: 12 (70.6%) anatomic, 3 (17.6%) imperfect, and 2 (11.8%) poor. The rate of anatomic reduction was significantly higher when sacroiliac separation was <= 0.5 cm (p=0.027). Conclusion: Associated posterior pelvic injuries, especially ipsilateral sacroiliac joint separation, accompany most transverse-oriented acetabular fractures and may influence the quality of acetabular reduction.