In recent years there have been increasing numbers of transcripts identified that do not encode proteins, many of which are developmentally regulated and appear to have regulatory functions. We have constructed a comprehensive mammalian noncoding RNA database (RNAdb) which contains over 800 unique experimentally studied noncoding RNAs (ncRNAs), including many associated with diseases and/or developmental processes. The database is available at http://research.imb.uq.edu.au/RNAdb and is searchable by many criteria. It includes microRNAs and snoRNAs, but not infrastructural RNAs such as rRNAs and tRNAs which are catalogued elsewhere. The database also includes over 1,100 putative antisense ncRNAs and almost 20,000 putative noncoding RNAs identified in high quality murine and human cDNA libraries, with more to be added in the near future. Many of these RNAs are large, and many are spliced, some alternatively. The database will be useful as a foundation for the emerging field of rnomics and the characterization of the roles of ncRNAs in mammalian gene expression and regulation.