Acute calf pain can be due to a variety of causes. The identification of patients requiring screening for deep vein thrombosis can be achieved from the history and physical examination. For emergency department use, initially noninvasive assessment with ultrasound scanning can not only demonstrate thrombosis but also other lesions, including popliteal cysts and masses producing venous compression. This is a cost-effective strategy suitable for the assessment of the ambulatory patient. Some subgroups of patients with proven deep vein thrombosis may require further investigation for the predisposing cause.