Prenatal exposure to testosterone is known to affect fetal brain maturation and later neurocognitive function. However, research on the effects of prenatal testosterone exposure has been limited by indirect measures of testosterone and small unrepresentative samples. This study investigated whether bioavailable testosterone (BioT) concentrations in umbilical cord blood are associated with expressive vocabulary development, in a large birth cohort. Cord blood samples were taken immediately after delivery and expressive vocabulary was measured at two years of age using the language development survey (LDS). BioT concentration significantly predicted vocabulary size in males (n=197), such that higher concentrations were associated with lower LDS scores, indicating smaller vocabulary. This relationship between BioT concentrations and vocabulary at aged 2 years was not observed in girls (n=176). Higher circulating prenatal testosterone concentrations at birth may be associated with reduced vocabulary in early childhood among boys. Copyright © 2012 Elsevier B.V. All rights reserved.