Cochrane collaboration has been developing since 1992 as an international network aiming at performing systematic reviews of available data on therapeutic effectiveness. The fundamental principles of this organisation are trying to avoid duplication of efforts, seeking the best reliability, using reproducible and quantitative synthesis techniques, offering constantly updated results. All health domains are progressively covered. The production unit in one domain is the review group. The Hypertension Cochrane Review Group (HTN CRG) has been officially registered on May 15th 1996. Information and products from the group are available through its news letter, through the Cochrane Library CD-ROM, regularly updated, and on the Internet (http://merece.uthscsa.edu/htncrg). The Hypertension Cochrane Review Group includes an editorial board (with an administrator and three editors), the authors of systematic reviews, internal and external reviewers. The geographic link is the San Antonio Cochrane Centre (Texas, USA). Invitations to participate have been sent to people interested in hypertension and who where known to the Cochrane collaboration, and to authors of previous reviews in hypertension. It is possible to collaborate with the HTN CRG through: performing a systematic review; reviewing protocols and systematic reviews; hand-searching medical journals; being a member of the editorial team. The first protocol for a systematic review edited by the group concerns antihypertensive treatment in the elderly, and is available in the 1996 and subsequent editions of the Cochrane Library. The group welcomes other reviews from domains awaiting registration, and collaborates with related domains review groups such as Diabetes CRG, or Stroke CRG. The group contributes to the effort of hand-searching medical literature, Pr Plouin being responsible for the Archives des Maladies du Coeur et des Vaisseaux. The second edition in 1996 of the Cochrane Library included 114 systematic reviews and 131 protocols, being the only media with similar objectives.