Involuntary weight loss is an important clinical condition that has not been extensively covered in the medical literature. Searching MEDLINE, we identified twelve case series in different regions of the world, mostly in developed countries. Three series included ambulatory patients, while nine studied patients from secondary care hospitals. A quarter of patients with involuntary weight loss had a cancer, and many had psychiatric diseases. Organic causes of involuntary weight loss are usually revealed by a basic evaluation, and a normal result is generally reassuring. In this case it is recommended a watchful surveillance instead of blind investigations, because the prognosis of IWL of undetermined cause is good. On the contrary, patients with involuntary weight loss caused by cancers have a severe prognostic due to the advanced stage of the disease. On the other way, these patients are easy to diagnose.