Attention-deficit hyperactivity disorder (ADHD) affects 2-6% of school-age children and is a precursor of behavioural problems in adolescence and adulthood. Underlying the categorical definition of ADHD are the quantitative traits of activity, impulsivity, and inattention which vary continuously in the population. Both ADHD and quantitative measures of hyperactivity are heritable, and influenced by multiple genes of small effect. Several studies have reported an association between clinically defined ADHD and the seven-repeat allele of a 48-bp tandem repeat polymorphism in the third exon of the dopamine D4 receptor gene (DRD4). We tested this association in a large, unselected birth cohort (n = 1037) using multiple measures of the hyperactivity phenotype taken at multiple assessment ages across 20 years. This longitudinal approach allowed us to ascertain whether or not DRD4 has a general effect on the diagnosed (n = 49) or continuously distributed hyperactivity phenotype, and related personality traits. We found no evidence to support this association.