Idiopathic chronic sleep onset insomnia (SOI) in children with Attention-Deficit/Hyperactivity Disorder (ADHD) shows typical characteristics of the delayed sleep phase syndrome and could, therefore, be considered a circadian rhythm sleep disorder. A variable number tandem repeat (VNTR) polymorphism of the clock gene PER3 is associated with the delayed sleep phase syndrome and, hence, may associate with ADHD-related chronic SOI as well. Here, we investigated an association between ADHD-related chronic SOI and the VNTR polymorphism of PER3 in 10 medication naive children with rigorously diagnosed ADHD and SOI (ADHD-SOI), and in 10 normal controls. Actigraphic sleep onset and sleep duration and salivary dim light melatonin onset (DLMO) were evaluated in ADHD-SOI. The 4-repeat allele frequency was lower in ADHD-SOI (0.65) than in normal controls (0.75) ( p =0.73) with an odds ratio of 0.62 (CI 0.16-2.4). In ADHD-SOI, mean (SD) DLMO (21:380:50 h), sleep onset (22:170:46 h), and sleep duration (9:260:41 h) were not significantly related to the 4-repeat allele frequency. The present findings suggest no association between ADHD-related idiopathic chronic sleep onset insomnia and the PER3 VNTR polymorphism.