A leg ulcer is a symptom and the treating physician needs to find out its origin by differential diagnostic approaches and procedures. The correct diagnosis leads to a specific therapy that ideally accelerates the healing of the ulceration. Identifying the pathogenesis of a leg ulcer is the first and main step towards healing. Although vascular diseases are the major causes of leg ulcers, one needs to consider, in addition to venous and arterial disorders, autoimmune, infectious, metabolic and neoplastic causes. The simple truth that one can only make a diagnosis that was considered holds particularly true in leg ulcers. The differential diagnostic considerations presented here appear in the daily routine of a dermatologist and the article provides help in diagnostic approaches and therapeutic decisions.