This study examined whether 19-month-old-infants' social understanding was related to their interaction behavior during dyadic cooperation with a peer. Toddlers' ability to predict others' action intentions was examined using a computerized experimental task. The children watched a series of stimulus movies in which an actor expressed her liking or disliking towards two different objects and then announced that she was going to grasp one of them. Toddlers' eye movements were registered, and it was examined whether they showed anticipatory looks to the object the model was going to grasp. During the dyadic cooperation task the toddlers interacted with an unfamiliar peer. Toddlers' interaction performance during cooperation was observed, and affiliative and antagonistic behaviors were coded. Intention understanding was positively correlated with affiliative behaviors, and negatively with antagonistic behaviors during the cooperation task. Measures of cooperation success were not related to toddlers' intention understanding. Toddlers' capability to understand others' intentions was thus closely associated with their peer interaction behavior, but now with their task performance.