This paper reviews current knowledge of genetic factors in the treatment of bipolar disorder. Most studies to date have investigated genetic differences between responders and non-responders to lithium. The most provocative findings have been obtained in studies of serotonin transporter gene. Another promising application of pharmacogenetic research is the identification of subgroups of bipolar disorder defined by response (or non-response) to pharmacological treatments. Such subgroups could be used to define more homogeneous patient populations for gene-mapping studies.