The involvement of the mesocorticolimbic dopamine system in behaviors that are compromised in patients with mood disorder has led to the investigation of dopamine system genes as candidates for bipolar disorder. In particular, the functional VNTRs in the exon III of the dopamine D4 (DRD4) and in intron I of the tyrosine hydroxylase (TH) genes have been investigated in numerous association studies that have produced contrasting results. Likewise, linkage studies in multiplex bipolar families have shown both positive and negative results for markers in close proximity to DRD4 and TH on 11p15.5. We performed a linkage disequilibrium analysis of the DRD4 and TH VNTRs in a sample of 145 nuclear families comprised of DSM-IV bipolar probands and their biological parents. An excess of transmissions and non transmissions was observed for the DRD4 4- and 2-repeat alleles respectively. The biased transmission showed a parent of origin effect (POE) since it was derived almost exclusively from the maternal meiosis (4-repeat allele maternally transmitted 40 times vs 20 times non-transmitted; chi(2) = 6.667; df = 1; P = 0.009; while paternally transmitted 26 times vs 21 times non-transmitted; chi(2) = 0.531; df = 1; P = 0.46). The analysis of TH did not reveal biased transmission of intron I VNTR alleles. Although replication of our study is necessary, the fact that DRD4 exhibit POE and is located on 11p15.5, in close proximity to a cluster of imprinted genes, suggests that genomic imprinting may be operating in bipolar disorder.