The core of our paperwork consists in determine a "matrix of symptoms", based on grey systems theory. The symptoms of a firm can be represented by economic-financial ratios, usually used by analysts to make predictions and give recommendations. The ability to create such a matrix of symptoms implies that given level of symptom's intensity we can determinate if the analyzed firm presents some "diseases". In this article, by diseases we understand the causes which generate anomalies in the company's activity. By using such a matrix of symptoms, the activity of the analyst can be really improved, the quality of his or her recommendations rising significantly. First part of our article describes the evolutions of the methods used for diagnosing the companies' diseases. The second part focuses on the methods of diagnosis belonging to the Grey Systems Theory, while the third one illustrates their applications infirm's activity analysis, including also a numerical example. We conclude our paper with some remarks over the role and the utility of the grey systems theory in diagnosing the firms' diseases.