One hundred and fourteen femora (75 male and 39 female) derived from a contemporary rural Guatemalan population were studied to test the ability of the minimum supero-inferior femoral neck diameter as a sex indicator. With the discriminant functions previously developed from North American modern populations, a maximum of only 36% correctly sexed femora was obtained, with correct percentages as low as 4%. A new discriminant function for the Guatemalan rural population is presented, with a total of 89.5% correctly classified individuals. It is suggested that poor physical growth of the rural Guatemalan population, due to a stressful environment, can explain part of the metric differences observed between the North American and rural Guatemalan populations. Am J Phys Anthropol 121:000–000, 2003. © 2003 Wiley-Liss, Inc.