In 29 adults with attention deficit hyperactivity disorder (ADHD) striatal dopamine transporter (DAT) availability was assessed by [(99m)Tc]TRODAT-1 SPECT and correlated with 3' VNTR polymorphism of the DAT gene on chromosome 5p15.3. Seventeen patients showed homozygosity for the 10-repeat allele, two homozygosity of the 9 allele and 10 were heterozygous (9-10). No statistically significant difference in DAT availability was found between patients with 10-10 carriers (DAT 1.28 +/- 0.34) and with at least one 9 allele (DAT 1.31 +/- 0.27); when smokers were excluded, DAT availability was 1.38 +/- 0.28 in the 10-10 carriers (n = 12) and 1.42 +/- 0.19 in the 9-10 and 9-9 carriers (n = 7). In conclusion, no higher striatal DAT was found in patients with homozygosity of the 10 allele of the DAT gene in this study. These results differ from a study in 11 Korean children with ADHD, in which 10-10 carriers showed higher DAT availability in [(123)I]IPT SPECT. Discrepancies may be explained by differences in patient's age, ethnical differences, different imaging techniques or the limited number of patients included in both studies.