A new standardized scoring method was used to study age variation of the degree of fusion of the ventral face of the sacral vertebral bodies (SVF) in 904 adult skeletons of both sexes from two identified modern samples (20th c.): Frassetto collections (Museum of Anthropology, University of Bologna, Italy) and Colecção de Esqueletos Identificados (Museum of Anthropology, University of Coimbra, Portugal). SVF was scored for each fusion site (between S1 and S2, etc.) on a four-stage scale, from nonfusion (Degree 0) to complete fusion (Degree 3). Inter-observer error of 1.2% suggested good reproducibility. The data were analyzed according to age, sex, and population. There were sex and population differences only in young adults (20–34 years). Sex differences in SVF were present in both collections, with females displaying earlier fusion than males. The differences occurred until 35 years, after which there was a generalized prevalence of completely fused sacra. Population differences were observed for males, with the Sassari men showing later sacral vertebral fusion than the Coimbra men. Our study suggests that incompletely fused sacra can help in distinguishing young adults from old adults regardless of sex and population. Am J Phys Anthropol 135:149–160, 2008. © 2007 Wiley-Liss, Inc.