Aim: It has been suggested that high levels of prenatal testosterone exposure are implied in the aetiology of attention-deficit-hyperactivity disorder (ADHD). This study examined the association between the ratio of the length of the second and fourth digits (2D:4D ratio), a marker of fetal testosterone exposure, and the presence of ADHD-related cognitive and behavioural problems in children with ADHD and in typically developing comparison individuals. Method: A clinically referred group of 64 children who fulfilled DSM-IV-TR criteria for ADHD (47 males, 17 females; mean age 8y 8mo, SD 1y 8mo, range 7-12y) and 46 comparison children (25 males, 21 females; mean age 9y 2mo; SD 1y 10mo, range 7-12y) were included in the study. The length of the second and fourth digits was measured by two independent raters. The Child Behaviour Checklist (CBCL) and the Test of Everyday Attention for Children (TEA-Ch) were used to assess behavioural problems and different aspects of attention. Results: No group differences in 2D:4D ratio were observed between children with (combined, inattentive, or hyperactive-impulsive subtype of) ADHD and comparison children. The ratio did not show the postulated relation with cognitive and behavioural aspects of ADHD. Interpretation: These findings challenge the hypothesis that fetal testosterone exposure plays a prominent role in the aetiology of ADHD.