The ratio of 2nd to 4th digit length (2D:4D) is sexually dimorphic (mean 2D:4D is lower in males than females) and is thought to be fixed early in development. 2D:4D has been reported to be related to fetal growth, hand preference, autism, Asperger's syndrome, sperm counts, family size, age at myocardial infarction in men and breast cancer in women. There is indirect evidence that 2D:4D is established in utero and is negatively related to prenatal testosterone and positively with prenatal estradiol. However, there are no studies which show direct relationships between fetal testosterone (FT), fetal estradiol (FE) and 2D:4D. To investigate the relationships between 2D:4D ratios and FT and FE from amniotic fluid. Cohort study. 33 children. Radioimmunoassays of FT and FE obtained from routine amniocentesis; 2D:4D ratios calculated from 2nd and 4th digit length of the right and left hands at age 2 years. A significant negative association between right 2D:4D ratio and FT/FE ratio, which was independent of sex. These preliminary findings lend support to an association between low 2D:4D and high levels of FT relative to FE, and high 2D:4D with low FT relative to FE.