Unintentional weight loss (UWL) was documented in 154 patients (2.8%) admitted to an internal medicine department during a two-year period. A retrospective analysis of the clinical charts showed that more than one third (36.3%) were found to have a neoplasm, involving preponderantly the gastrointestinal tract. Patients with neoplasia were older and more frequently had abnormal physical findings and significantly lower values of serum albumin as well as higher values of alkaline phosphatase than other patients. Despite extensive investigations, in 36 patients (23.3%) UWL remained unexplained even after prolonged follow-up periods. The remaining 62 patients had a variety of disorders, preponderantly gastrointestinal tract (26 patients) and psychiatric (16 patients) diseases. Endocrinologic disorders such as diabetes mellitus and hyperthyroidism were relatively uncommon (3.8%). Unintentional weight loss is a relatively common problem in clinical practice. Familiarity with the spectrum of disorders associated with UWL as well as selected clinical and laboratory values may help in the initial examination of these patients.