The genes encoding for the enzymes monoamine oxidase (MAO) A and B are good candidates to investigate bipolar affective disorder. A 30 bp repeat in the MAOA promoter was recently demonstrated to be polymorphic and to affect transcriptional activity. In a family-based association design we found that none of the different repeat copies was preferentially transmitted from mothers (n = 131) to their children affected with bipolar disorder (chi(2) = 2.75, 4 d.f., p = 0.6). Following on our previous finding of an excess of low-activity genotypes of catechol-O-methyltransferase in patients with a rapid cycling form of illness, we examined for a similar trend with MAOA alleles. In an extended sample we found a non-significant trend for patients with an ultra-rapid cycling form of illness (n = 29) to have a higher frequency of low-activity alleles compared with 92 bipolar patients with a non-rapid cycling course of illness (chi(2) = 2.37, 1 d.f., p = 0.13).