The purpose of this study was to examine the extent to which perceptions of surface-level (i.e. age, ethnicity, physical condition) and deep-level (i.e. attitudes, beliefs, values) similarity, as assessed shortly after group formation, predict participants' subsequent attendance behaviors within postnatal physical activity classes. Women (N = 136) enrolled in 21 postnatal exercise classes participated in this prospective observational study. Perceptions of surface-level similarity, in particular with regard to other group members' ages, predicted women's subsequent adherence to their respective exercise classes. The results of this study suggest that perceptual similarity may have noteworthy implications for mothers' involvement within postnatal physical activity classes.