This introductory guide represents an operative tool to conduct epidemiological studies in the area of comparative outcomes evaluation. It is based on the experience of epidemiological research in this field conducted in Italy within national (BPAC-Esiti del bypass aortocoronarico, Progetto mattoni outcome, Progetto Progressi) or regional (P.Re.Val.E. Programma Regionale di Valutazione degli Esiti, Lazio) health care outcomes projects and the National outcome programme. This guide is aimed to all those interested in conducting or interpreting health care outcomes studies within different levels of the Italian NHS. It gives an introductory description of the operative steps to build outcome indicators and to perform comparative analyses, with the general objective of measuring and promoting improvement in health care. A specific emphasis is given to the use of routinely collected health care databases that have found widespread use for epidemiological purposes. This guide has two parts: part A includes an introduction and comments on critical methodological points, part B shows three example of epidemiological studies (A. Complications after cholecystectomy: comparison between two surgical techniques, B. 30-day mortality after acute myocardial infarction: comparison among hospitals, C. 30-day mortality after acute myocardial infarction: comparison between time periods). The online version of this guide is organised as a hypertext as practical instrument of appraisal.