Child-child similarity on attachment and temperament were examined, in turn, as predictors of interaction quality between previously unacquainted children. At 33 months, child-mother attachment security was assessed, and parents reported on child temperament. At 39 months, 114 children were randomly paired into 57 same-sex dyads and observed during 3 laboratory visits over a 1-month period. Positive interaction (composed of ratings of dyadic coordination, social play complexity, and shared positive affect) was assessed from recordings of play sessions at each visit. Multilevel models revealed that child-child similarity on (a) attachment security predicted more rapid increases in positive interaction across the 3 visits for dyads averaging high security, (b) temperamental pleasure predicted more positive interaction, on average, for dyads averaging moderate to high pleasure, and (c) temperamental anger and fearfulness yielded equivocal results. Developmental and methodological implications of investigating child-child similarity on attachment and temperament as a window into the acquaintanceship process among young children are considered.