BACKGROUND: Autism spectrum conditions have been characterised as an extreme presentation of certain male-typical psychological traits. In addition, several studies have established a link between prenatal exposure to testosterone and cognitive sex differences in later life, and one study found that foetal testosterone (FT) is positively correlated to autistic traits in 6 to 10 year-old children. In this study, we tested whether FT is positively correlated with autistic traits in toddlers aged 18-24 months. METHODS: Levels of FT were analysed in amniotic fluid and compared with autistic traits, measured using the Quantitative Checklist for Autism in Toddlers (Q-CHAT) in 129 typically developing toddlers aged between 18 and 24 months (mean +/- SD 19.25 +/- 1.52 months). RESULTS: Sex differences were observed in Q-CHAT scores, with boys scoring significantly higher (indicating more autistic traits) than girls. In addition, we confirmed a significant positive relationship between FT levels and autistic traits. CONCLUSIONS: The current findings in children between 18 and 24 months of age are consistent with observations in older children showing a positive association between elevated FT levels and autistic traits. Given that sex steroid-related gene variations are associated with autistic traits in adults, this new finding suggests that the brain basis of autistic traits may reflect individual differences in prenatal androgens and androgen-related genes. The consistency of findings in early childhood, later childhood and adulthood suggests that this is a robust association.