BACKGROUND Measuring the perception of friendship in adults with Down syndrome (DS) has long been a research challenge. While there have been studies investigating the number of friends children with DS have in, the study of how adults with DS view the concept of friendship has been relatively unexplored. The aim of this study was to evaluate the perception of friendship in adults with DS using a visually based scale. METHODS Sixty-six individuals participated in this study: 22 adults with DS, 22 typical mental age (MA) matched children and 22 typical adults matched for chronological age (CA). We administered a visually based Friendship scale made up of photographs depicting social interactions between individuals or groups. The scale was composed of two parts. In Part 1 participants were shown two photographs and asked to select the photograph that best depicted friends. In Part 2 participants were asked to view one photograph and asked, 'Is it okay for friends to do this?' RESULTS Adults with DS scored lower on the Friendship scale in comparison with the CA and MA matched groups. Adults with DS made more errors in identifying 'friends' from 'non-friends' but were equally able to distinguish friendly behaviours and actions from non-friendly behaviours as their CA and MA matched peers. Individuals with DS were more likely to incorrectly identify photographs depicting a teacher, or a mother with a child as friends. Actions or behaviours that depicted subtle negative emotions were also incorrectly identified. CONCLUSIONS These results are an important first step in understanding the perception of friendship and social behaviours related to friendship in adults with DS.