There are no detailed anatomic studies focusing on the posterior iliac crest although it frequently is used for posterior stabilization of unstable pelvic fractures. Anatomic dissections were done to evaluate the size of the extraarticular region of the posterior iliac crest and its relationship to the lumbosacral lamina and to show on cadavers the level of sacral bar placement that offers safe and solid fixation. Sixty cadavers were dissected bilaterally. Fifty-one were male and nine were female. The distance between the posterior wall of the sacral canal and the tip of the iliac crest was measured at various levels between the level of the upper border of L5 lamina to the level of the posterosuperior iliac spine. In all the dissections the greatest distances were at the level of the L5-S1 junction, which consequently is the safest level for good bony purchase. The entire length of the posterior iliac crest from the level of the upper border of L5 lamina to the posterosuperior iliac spine was shown to be appropriate for safe and solid bar fixation because all of the distance measurements were greater than 13 mm, which is the smallest safe distance. Below the posterosuperior iliac spine level, insertion of the sacral bars was dangerous because the average measured distance was only 10.38 mm.