Countershading is often thought to be an adaptation for increasing crypsis, yet few quantitative studies have examined this assumption. A recent study showed that large primates display weaker countershading compared with small species, possibly due to a reduced predation risk. In addition to body mass, other factors likely affect countershading intensity. We predicted that if countershading is related to crypsis, then countershading intensity should be negatively related to the frequency of being in a vertical postural position because dorsoventral countershading is most effective when an animal adopts a horizontal position. In addition, countershading intensity may be positively related to group size if individuals are more conspicuous living in large groups or negatively related to group size if countershading further enhances a cryptic life style. We used color-corrected digital photographs of museum skins to quantify the luminance values of the ventral and dorsal surfaces of 113 primate species. We analyzed these data in a multiple regression using phylogenetically independent contrasts. While accounting for body mass, we found a significant negative relationship between the degree of countershading and the frequency of being in a vertical postural position. In contrast, we did not find a strong effect of group size on countershading. Our results suggest that countershading is weak or absent in species of any size that often adopt vertical postural positions because a crypsis benefit is only gained when being horizontal. Finally, the increased conspicuousness of species in large groups does not have a major effect on countershading intensity.