Unexpected weight loss is a symptom that can be associated with cancer, but also with a range of other conditions – both benign and serious. This can result in uncertainty regarding which patients should be investigated further, and those that can be spared unnecessary investigation. The degree of weight loss that best indicates underlying cancer in primary care is poorly defined. This article describes the current evidence regarding how much weight loss, over how much time, in combination with what other clinical features makes cancer sufficiently likely to warrant urgent investigation in primary care.Â