The purpose of this paper is to contribute to the current understanding of the variability of complete sacral clefts in human populations by presenting new data on a large prehistoric and historic Native American skeletal sample (n = 1943). Results are examined by age, sex, time period, and regional distribution and compared with reported frequencies of complete sacral clefts in other modern, historic, and ancient populations. In all, 1.6% of the sample exhibited complete sacral clefts, including 2.1% of males and 1.1% of females. Although males exhibit a frequency twice as high as females, this difference is not statistically significant. However, within the Alaskan sample, the sex difference was significant (p = 0.002), with 3.4% of males and 0.5% of females exhibiting complete clefts; these differences may be related to mechanical influences during growth and development in males. Differences among age groups are not significant. Regional comparisons among Alaska, Eastern Woodlands, Great Basin/Northwest/California, Great Plains, and Southwest showed no significant differences overall, but Alaska (2.2%) and Great Plains (0.5%) do show a significant difference (p = 0.024). No significant differences were found between prehistoric, protohistoric, and historic/recent samples, suggesting a lack of a secular trend in frequency of complete sacral clefts in Native American populations. One individual exhibited an enlarged canal, which may be indicative of a more severe condition. Copyright © 2012 John Wiley & Sons, Ltd.