BACKGROUND Testosterone is the key hormone for the development of male reproductive organs and secondary sexual characteristics. In addition, testosterone is associated with behavioural traits, including sexual behaviour and social dominance. The level of circulating testosterone in the human body is determined by genetic and environmental factors. Twin studies have shown moderate to high heritability in adolescence and adulthood, but heritability in early childhood has not been investigated. This study aimed at disentangling the genetic and environmental contributions to testosterone levels soon after birth. METHODS Using a sample of 314 twin pairs, saliva testosterone levels were measured at 5 months after birth. Quantitative genetic modelling was used to assess genetic and environmental contributions to the variation in testosterone levels. RESULTS Variation in testosterone levels was explained by common (56.6%) and unique (43.4%) environmental factors. CONCLUSIONS Taken together, these data from the largest study of twin testosterone levels suggest that, in contrast to findings in adulthood, environmental factors determine the interindividual variability in testosterone levels in early infancy. This may have consequences for the development of sex-related behaviour during childhood and calls for studies designed to unravel specific genetic and environmental factors involved in this process.