Lithium is the most effective mood-stabilizing drug in the therapy of bipolar affective disorder (BP). It is thought to exert its effect via the phosphatidylinositol signalling system. Myo-inositol monophosphatase 2 (IMPA2) codes for an enzyme in this system that is inhibited by lithium. It is located on 18p11.2, a region implicated as a BP susceptibility locus. We examined eight single-nucleotide polymorphisms (SNPs) identified within this gene for association with BP, using 237 parents-offspring trios and in 174 cases and 170 controls. No SNP showed association with BP. When good responders to lithium treatment were compared with the poor responders, some statistically significant differences emerged for two SNPs; however, the sample became too small to draw definitive conclusions. We cannot find support for the involvement of variation in IMPA2 in susceptibility to bipolar disorder, but the role of this and other genes from the phosphoinositol signalling pathway in predicting response to lithium treatment merits further investigation.