Positron emission tomography (PET) studies have revealed significant interindividual variation in dopamine D 2 receptor density in vivo in human striatum. 1 Low D 2 receptor binding in vivo has been found to associate with alcohol/substance dependence. 2–6 It has been suggested that the A1 allele of human D 2 receptor gene might be associated to a specific type of alcoholism 7 and possibly to a reduced D 2 receptor density in vitro. 8 We have determined D 2 dopamine receptor-binding density (B max), affinity (K d) and availability (B max/K d) in 54 healthy Finnish volunteers using PET and [11 C] raclopride in order to determine whether the A1 allele is associated with a ‘baseline’difference in D 2 receptor characteristics in vivo. A statistically significant reduction in D 2 receptor availability reflecting an alteration in receptor density was observed in the A1/A2 genotype group compared to the A2/A2 group. There was no difference in apparent K d between the two groups. In conclusion, the association between the A1 allele and low D 2 receptor availability in healthy subjects indicates that the A1 allele of the TaqIA polymorphism might be in linkage disequilibrium with a mutation in the promoter/regulatory gene element that affects dopamine D 2 receptor expression. This study provides an in vivo neurobiological correlate to the A1 allele in healthy volunteers.