Weight loss can herald the existence of an underlying serious disease. The purpose of this study was to ascertain the frequency of the involuntary weight loss at the Internal Medicine inpatient service of our Institution and the different conditions associated with it. The study was undertaken from March 1984 to February 1985. All patients who had lost at least 10% of their total body weight 6 months prior to admission were included. Those patients who had an evident organic disease, which could explain their weight loss, were excluded. A complete clinical history was performed in all cases; additional data concerning family and employment aspects and symptoms of depression were recorded. The evaluation also included screening laboratory examinations and specific diagnostic procedures. The study group consisted of 50 patients, 32 males and 18 females whose average age was 57.6 and 62.5 years, respectively. Mean average percentage of weight loss was 19%. The involuntary weight loss turned out to be clearly related to an organic cause in only 17 patients (34%), five of which had a malignant neoplasm. On the other hand, in 21 cases, factors such as stress inductors and depression seemed directly related to the involuntary weight loss. It is concluded that malignant diseases are not the pathology most frequently associated with unexplained involuntary weight loss in our Institution. Differential diagnosis must consider other organic and psychiatric disorders. Particular attention should be paid to those data relative to dietary habits, the patient's family and symptoms of depression in order to attain a more integral view of the patients.