Current guidelines for selecting lineup fillers are imprecise. Consequently, filler characteristics likely vary widely across lineups, potentially affecting identification decisions in important but unexplained ways. We report 2 experiments investigating the impact of 1 source of variation, the number of relatively high versus low similarity match-description fillers, on identification outcomes. Identifications following a retention interval of a few minutes (Experiments 1 and 2) and several weeks (Experiment 2) were examined. Increasing the number of high similarity lineup members within match-description lineups increased choosing (characterized by more filler but fewer suspect identifications), decreased accuracy and confidence and caused a poorer confidence-accuracy relationship. The changes to identification outcomes as the number of high similarity fillers increased were attributable to decreased discriminability and a relaxed criterion, with positive identifications requiring a lesser evidential discrepancy between target and fillers. There was limited evidence of guilty and innocent suspect identifications being differently affected by variations in the number of high similarity fillers and no evidence of suspect discriminability being influenced. For decisions after a short retention interval, high confidence suspect identifications were good predictors of accuracy; however, high similarity match-description fillers undermined the predictive value of high confidence suspect identifications at the long retention interval. Our results suggest that future research must explore methods for curbing variation in identification patterns resulting from uncontrolled filler characteristics. Until then it is critical that identification evidence be interpreted acknowledging that, even in lineups constructed following best practice guidelines, filler characteristics could have profoundly influenced the outcome. (PsycInfo Database Record (c) 2021 APA, all rights reserved)