The transgenic knockout mouse is one of the most important tools of modern biology, and commonly employed by pain researchers to examine the function of genes of interest. Over 400 papers, at a current rate of >60 papers per year, have been published to date describing a statistically significant behavioral pain "phenotype" resulting from the null mutation of a single gene. The standard literature review format is incapable of providing a sufficiently broad and up-to-date overview of the field. We have therefore constructed the Pain Genes Database, an interactive, web-based data browser designed to allow easy access to and analysis of the published pain-related phenotypes of mutant mice (over 200 different mutants at the date of submission). Manuscripts describing results of pain-relevant knockout studies were identified via Medline search. Manuscripts were included in the database if they described the testing of a spontaneous or genetically engineered mutant mouse with null expression of a single gene on a behavioral assay of acute or tonic nociception, injury- or stimulus-induced hypersensitivity (i.e., allodynia or hyperalgesia), or drug- or stress-induced inhibition of nociception (i.e., analgesia), and reported at least one statistically significant difference between the mutant mice and their simultaneously tested wildtype controls. The database features two levels of exploration, one allowing the identification of genes by name, acronym, genomic position or "summary" phenotype, and the other allowing in-depth browsing, paper-by-paper, of specific phenotypes and test parameters. Links to genetic databases and Medline abstracts are provided for each gene and paper. It is our intention to update the database continually based on weekly Medline searches. This database should provide pain researchers with a useful and easy-to-use tool for the generation of novel hypotheses regarding the roles of genes and their protein products in pain processing and modulation. It can be accessed at http://paingeneticslab.ca/4105/06_02_pain_genetics_database.asp (or by visiting paingeneticslab.ca and clicking on the "Pain Genes Db" link under "Resources").