In this article we describe the causes and diagnostic work-up of unintentional weight loss. It is defined as loss of weight of at least 5% in 6 months. There is both attention to somatic and functional causes of weight loss, as well as iatrogenic causes like side-effects of medication. Referrals to medical specialists for diagnostics are discussed and also the role of dieticians in screening and treatment. Special attention is given to weight loss by malabsorption. Although malignancies as a reason of weight loss is rare in primary care, in secondary care it may be present up to 30%. Treatment of unintentional weight loss may be necessary by high caloric dietary foods when functional decline is obvious, although there is hardly indication for prescribed medication, guidelines for treatment is given.