BACKGROUND We investigated the relationship between patterns of sex and hand differences in circadian wrist activity and digit ratio, a marker for prenatal androgen exposure. If the contribution of prenatal androgen exposure to sex differences in digit ratio underlies sex differences in circadian wrist activity, we predict that patterns of wrist activity will be correlated with digit ratio. METHODS Bilateral wrist activity of male and female college students was measured for three consecutive days. Digit ratio was obtained from photocopy measurements of the second and fourth fingers of each subject. RESULTS Males had lower digit ratios with more pronounced differences on the right hand. Female acrophase occurred earlier than male acrophase. There was more activity in the right hand and right hand activity peaked before the left. Digit ratio was not correlated with any measure of wrist activity. An analysis of activity by age revealed that younger female students exhibited more male-like activity patterns. CONCLUSIONS Sex and hand differences for digit ratio and acrophase replicated previous findings. The lack of correlation between digit ratio and patterns of wrist activity suggests that sexually dimorphic circadian activity develops independently from the mechanisms of hormone exposure that cause sex differences in digit ratio.