The present study surveys autistic behavioral characteristics of 58 mentally retarded male adults with fragile-X syndrome. Results were compared with a control group of 58 fragile-X negative mentally retarded men, matched on age, cognitive level, living conditions and length of institutionalisation. We demonstrate that this and other controlled studies have not given much evidence for a specific association between fra(X) syndrome and the global diagnosis of autism. A high proportion of fra(X) adults display a limited number of specific behavioral characteristics that belong to the spectrum of pervasive developmental disorders. Non-functional body movements, unacceptable vocal and speech habits, social avoidance reactions and eccentric peculiarities are significantly different between the two groups. We further propose that the core features of autism. namely social indifference and severely disturbed social relations, are in general not found in the fra(X) group. On the contrary, they had a more positive general mood, were more sensitive to social contact and attention from others and approached not only new physical environments but also other people with great openness and interest. Moreover, we found that fra(X) adults had average interpersonal and communicative skills. This peculiar combination of <<autistic-like>> behavior and social sensitivity or openness could probably be accounted for by feelings of social anxiety. Finally implications of this <<social anxiety>> hypothesis for research and treatment are discussed.