Relations between having an imaginary companion (IC) and (i) descriptions of a real-life friend, (ii) theory of mind performance, and (iii) reported prosocial behaviour and behavioural difficulties were investigated in a sample of 5-year-olds (N = 159). Children who had an IC were more likely than their peers without an IC to describe their best friends with reference to their mental characteristics, but IC status was unrelated to children's theory of mind performance and reported prosocial behaviour and behavioural difficulties. These findings are discussed in the context of the proposal that there is a competence–performance gap in children's mentalizing abilities. © 2014 The Authors. Infant and Child Development published by John Wiley & Sons Ltd.