Clinically‐important involuntary weight loss is usually defined as >5 percent of body weight lost over 6–12 months. It often indicates a serious underlying medical or psychiatric illness. Changes in medication or diet and chronic obstructive airways disease are common explanations and often overlooked. Basic diagnostic evaluation should include thorough history and physical examination, routine hematology and biochemistry screen, thyroid fuction test, erythrocyte sedimentation rate, C‐reactive protein, Celiac antibody test, chest radiograph and upper gastrointestinal endoscopy with duodenal biopsy. Some tumour markers (CA‐19.9, AFP, CA125) can be included but need interpreting with caution. If initial tests are negative, abdominal scanning, preferably PET/CT, should be performed and should include the pelvis in females. If no abnormality is identified after a thorough initial evaluation, it is uncommon for a serious explanation to become apparent later and the patient can be reassured and reassessed in 1–3 months.