In the last 30 years the advanced countries experienced a large growth of financial and social resources devoted to Health Care (HC) [1] The need of better co-ordinating activities of a composite large set of specialists, and the need of avoiding further growth of financial and social resources without reducing benefits, stimulated the growth of Health Care Evaluation (HCE) and Quality Assurance (QA) studies. In this paper we show how system analysis and precise assessment of the clinical process can be largely enhanced by using formal techniques. The proposed methodology takes advantage of positive experiences in computer science. It is based on a formal model of the clinical process that provides the physicians with support for assessing diagnostic procedures in a co-operative surgical environment [7]. Diagnostic, grading, and staging procedures are integrated in a single model that can be formally expressed and analyzed. The approach is exemplified by presenting an elementary model of the diagnosis, grading and staging procedures for the colo-rectal cancer and ulcerative colitis.