The concept of pharmacogenomics, the study of how variation in the human genome affects response to drugs, attracts attention from clinicians and the pharmaceutical industry alike. The aim is to distinguish, using appropriate genetic tests, individuals who may be harmed by certain drugs from those who may benefit from them. Adverse drug reactions cause significant morbidity and mortality and incur a large cost to healthcare systems. Pharmacogenomics may help in the prediction and prevention of adverse reactions to drugs. While some recent studies (e.g., abacavir hypersensitivity) have shown strong associations with single genetic factors, whether these represent the exceptions rather than the rule is unclear. Further studies on adverse drug reaction pharmacogenetics are needed - these should be adequately powered and utilize the most appropriate study design that allows for an evaluation of both genetic and environmental factors. For pharmacogenetic testing to become acceptable in clinical practice, it is important that such studies are also able to provide evidence of clinical validity and clinical utility.