In this paper the authors have evaluated the incidence and the clinical implications of sick euthyroid syndrome (SES) in a group of 144 patients in a department of internal medicine. SES is an alteration of thyroid hormone values in the absence of a thyroid disease, which is seen in patients suffering from serious diseases. Having classified SES into 3 subgroups according to the different alterations seen in the values of T3, T4, FT3, FT4, TSH, rT3 and TBG, they show the hypotheses that explain the biochemical mechanisms which are at the basis of these hormonal alterations. Fourteen of the 144 patients under observation were excluded as they were suffering from ascertained or subclinical thyroid disease. Thirty (23% of cases) of the remaining 130 patients had alterations of the thyroid hormones in accordance with SES diagnosis. Of these 30 patients, 19 had hormone values found in SES type I (63%), 2 in SES type II (6.5%) and 9 in SES type III (30.5%). In SES type I the diseases seen, in order of frequency, were: obstructive chronic bronchopneumopathy with acute respiratory failure, diabetic ketoacidosis, neoplasms, ischemic heart disease, cardiac failure, chronic renal failure, liver diseases, acute cerebral vasculopathies, sepsis and collagenopathies. The disease seen in the 2 cases of SES type II was obstructive chronic bronchopneumopathy with acute respiratory failure. In SES type III the diseases seen were, in order of frequency: diabetic ketoacidosis, lung diseases, ischemic heart disease, cardiac failure, peripheral arteriopathies, acute cerebral vasculopathies, neoplasms, liver diseases, acute renal failure. The incidence of SES in 23% of the admitted to hospital patients was found to be slightly higher than in other studies; this could be explained by a stricter selection of inpatients: in fact self-sufficient patients or those not needing urgent admission, were sent to an efficient out patient clinic where necessary examinations were quickly carried out, hospitalization being reserved for patients with more serious illnesses. We would like to underline how the incidence of SES is much greater than that of what is known as thyroid disease (23% compared to 5%), thereby confirming that it is the most frequent cause of alterations of thyroid hormones. With regard to the pathogenetical hypotheses, it is confirmed that in SES, the reduction of T3 values is accompanied by an increase in the values of rT3 as for reduced activity of 5-desiodinasis enzyme. In SES type III the increase of T4 values is due to the increase of TBG resulting in an increase in the link for T4 and therefore a reduced peripheral hormone activity.(ABSTRACT TRUNCATED AT 400 WORDS)