Background: Sex-differences exist in some areas of human social behaviour. In animals, foetal testosterone (fT) plays a central role in organising the brain and in later social behaviour. fT has also been implicated in language development, eye-contact, and spatial ability in humans. Methods: Fifty-eight children (35 male and 23 female), whose fT was analysed in amniotic fluid, were followed up at age 4. Their mothers completed the Children's Communication Checklist, a questionnaire assessing language, quality of social relationships and restricted interests. Results: fT was negatively correlated to quality of social relationships, taking sex-differences into account. fT was also positively correlated with restricted interests in boys. Conclusions: These findings implicate fT in both social development and attentional focus. They may also have implications for understanding the sex ratio in autism.