Microarrays are simple assays that measure the relative expression levels of tens of thousands of genes. Excitement about their importance and potential contributions to biology and medicine has been intense. Nonetheless, recent insights into the limitations and pitfalls of microarrays have led to caution about data interpretation. Microarrays are very useful but they are also very misleading; better data analysis tools are needed to improve accuracy. ©2005 American Medical Association. All rights reserved.