Background Alcohol intake during pregnancy is considered to be a risk factor for child development. Child biomarkers of intrauterine alcohol exposure have been rarely studied. We investigated whether a meconium alcohol metabolite (ethyl glucuronide, EtG) was associated with cognitive development, ADHD‐related behaviour and neurophysiological markers of attention and executive control of children at primary‐school age. Methods Mothers provided self‐report on prenatal alcohol consumption during their 3rd trimester. Meconium samples were collected at birth. A total of 44 children with a meconium EtG above the detection limit (≥10 ng/g) and 44 nonexposed matched controls were compared. A second threshold (≥154 ng/g) was applied to study the dose effects. When children reached primary‐school age, mothers rated ADHD‐related behaviour, child cognitive development was measured using an IQ test battery, and event‐related potentials were recorded during a cued go/nogo task. Results Children in both EtG‐positive groups allocated fewer attentional resources than controls to the go/nogo task (reduced P3 component in go‐trials). Children with a meconium EtG above 154 ng/g were also found to have an IQ that was six points lower than the other groups. Within the EtG ≥ 154 ng/g group, there was a positive correlation between EtG value and ADHD‐related behaviour. These significant effects were not observed in relation to the maternal self‐report data. Conclusions Associations between EtG and cognitive deficits, attentional resource capacity and ADHD‐related behaviour could be documented with effects that were partially dose‐dependent. In addition to maternal self‐reports, this biomarker of intrauterine alcohol exposure may be considered as a predictor of child development.