Motivation: SAGE enables the determination of genome- wide mRNA expression profiles. A comprehensive analy- sis of SAGE data requires software, which integrates (sta- tistical) data analysis methods with a database system. Furthermore, to facilitate data sharing between users, the application should reside on a central server and be ac- cessed via the internet. Since such an application was not available we developed the USAGE package. Results: USAGE is a web-based application that com- prises an integrated set of tools, which offers many functions for analysing and comparing SAGE data. Additionally, USAGE includes a statistical method for the planning of new SAGE experiments. USAGE is available in a multi-user environment giving users the option of sharing data. USAGE is interfaced to a rela- tional database to store data and analysis results. The USAGE query editor allows the composition of queries for searching this database. Several database functions have been included which enable the selection and com- bination of data. USAGE provides the biologist increased functionality and flexibility for analysing SAGE data. Availability: USAGE is freely accessible for academic institutions at http:// www.cmbi.kun.nl/ usage/ . The source code of USAGE is freely available for academic institutions on request from the first author.