Although the sequential lineup has been proposed as a means of protecting innocent suspects from mistaken identification, little is known about the importance of various aspects of the procedure. One potentially important detail is that witnesses should not know how many people are in the lineup. This is sometimes achieved by backloading the lineup so that witnesses believe that the lineup includes more photographs than it actually does. This study aimed to investigate the effect of backloading on witness decision making. A large sample (N = 833) of community-dwelling adults viewed a live "culprit" and then saw a target-present or target-absent sequential lineup. All lineups included 6 individuals, but the participants were told that the lineup included 6 photographs (nonbackloaded condition) or that the lineup included 12 or 30 photographs (backloaded conditions). The suspect either appeared early (Position 2) or late (Position 6) in the lineup. Innocent suspects placed in Position 6 were chosen more frequently by participants in the nonbackloaded condition than in either backloaded condition. Additionally, when the lineup was not backloaded, foil identification rates increased from Positions 3 to 5, suggesting a gradually shifting response criterion. The results suggest that backloading encourages participants to adopt a more conservative response criterion, and it reduces or eliminates the tendency for the criterion to become more lenient over the course of the lineup. The results underscore the absolute importance of ensuring that witnesses who view sequential lineups are unaware of the number of individuals to be seen.