Attention deficit hyperactivity disorder (ADHD) is a common childhood psychiatric disorder which affects between 3% and 5% of school aged children. Despite much research, little is known regarding the aetiology of the disorder. Maternal cigarette smoking during pregnancy has been linked to a number of negative effects in offspring in infancy, childhood and even into adulthood and has been proposed as a possible risk factor for ADHD. The aim of this review was to discuss the evidence associating maternal smoking during pregnancy and ADHD as well as methodological issues concerning this association. A literature search using PubMed was employed using relevant keywords. The relevant reference sections of articles found were also searched. All English language studies published before June 2005 were assessed. A pooled odds ratio derived from case-control studies was also obtained. Despite methodological limitations, the majority of studies identify maternal smoking during pregnancy as a risk factor for ADHD behaviours. A pooled odds ratio indicates more than a two-fold increase in risk for a diagnosis of ADHD in those individuals whose mothers smoked during pregnancy (odds ratio 2.39, 95% confidence intervals 1.61, 3.52 P<0.001). Maternal smoking during pregnancy is a risk factor for ADHD behaviour and diagnoses, although the mechanisms through which such risks work is unknown.