The current enthusiasm for pharmacogenetics draws much of its inspiration from the relatively few examples of polymorphisms that have marked and seemingly clinically relevant effects on drug response. In this regard, pharmacogenetic research has paralleled the study of human disease, which has enjoyed success in identifying mutations underlying mendelian conditions. Progress in deciphering the genetics of complex diseases, involving the interaction of multiple genes with each other and with the environment has been considerably less successful. In most instances, drug responses will probably also prove to be complex, influenced by both the environment and multiple genetic factors. For pharmacogenetics to deliver on its potential, this complexity will need to be recognized and accommodated, both in basic research and in clinical application of pharmacogenetics. As the attention of researchers begins to shift toward more systematic pharmacogenetic investigations, we suggest some priorities and standards for pharmacogenetic research.