Recently, an interaction between season of birth and the expression of candidate genes has been suggested. Season of birth variations in tryptophan hydroxylase (TPH), the serotonin transporter (5-HTTLPR) and the dopamine D4 receptor (DRD4) gene polymorphisms are different for affective disorders and schizophrenia. The DRD4 gene has been postulated as a candidate gene for attention-deficit-hyperactivity disorder (ADHD), equivalent to hyperkinetic disorder (HD). The seven-repeat long variant of this gene (DRD4*7) in comparison to the short repeat variants of the DRD4 gene polymorphism, has been found to be associated with ADHD. A seasonal pattern of birth has also been proposed for different subtypes of ADHD. Therefore, in a subgroup of children with HD and conduct disorder (CD) and in healthy controls, we investigated a possible association between the DRD4*7 allele and HD + CD in association with the season of birth. Supporting this hypothesis, we found an interaction between the seasons of birth and the expression of the DRD4 candidate gene in children with HD + CD as well as in controls, which differ significantly from each other. Depending on the season of birth, children carrying the DRD4*7R allele showed different relative risks for developing HD + CD.