Objective: To investigate a possible association between a 3'UTR VNTR polymorphism of the dopamine transporter gene (SLC6A3) and ADHD in a Brazilian sample of adult patients. Method: Study Case-control with 102 ADHD adult outpatients ("DSM-IV" criteria) and 479 healthy controls. The primers' sequence used were: 3'UTR-Forward: 5' TGT GGT GAT GGG AAC GGC CTG AG 3' and 3'UTR-Reverse: 5' CTT CCT GGA GGT CAC GGC TCA AGG 3'. Alleles of the 3'UTR were coded according to their number of repeats: 6- repeat 320 bp (allele 6), 8- repeat 400 bp (allele 8), 9- repeat 440 bp (allele 9), 10- repeat 480 bp (allele 10), and 11- repeat 520 bp (allele 11). Results: There were no allelic (chi[superscript 2] = 2.67, 5df, p = 0.75) and genotypic (chi[superscript 2] = 7.20, 1df, p = 0.61) association between adult ADHD and VNTR 3'UTR polymorphism of SLC6A3. Conclusion: Our findings do not support SLC6A3 as marker genetic susceptibility factor in adult ADHD. More comprehensive polymorphism coverage within the SLC6A3 region should be conducted in larger samples, including comparisons in clinical subgroups, and in samples with different ethnic backgrounds. (Contains 1 table.)