This report describes a computer based program of patient clinical data collection: the ARCHIDIA system. The project relies on descriptive analysis of clinical events according to well defined methodological criteria. This allows the formulation of a concise diagnosis which is, at the same time, exhaustive of all essential information. Two are the basis principles of this methodology: To define, as accurately as possible, the logical steps necessary to elaborate the diagnosis, that is construed by a sequence of codes. To define all the conditions that must be followed so to use any code in a controlled and independent way. These criteria were derived from literature. The major claim of the system is likely to be the introduction of a "common language" between different ICUs. Uniformed diagnostic and clinical criteria are the main source of large data collection for descriptive, analytic and prospective studies. After a one year pilot study performed by 4 ICUs, ARCHIDIA was used, in 1991, by 20 centers from the area of Milan, Pavia, Como, Varese (70% of total) and 4148 patient data were collected. A descriptive analysis will be reported in the following paper.