A high index for compartment syndrome should be maintained in all patients with an injured upper extremity. This is particularly important in obtunded patients and any other patients whose ability to communicate is impeded. Such patients include victims of severe or multiple trauma, particularly to the head, spinal cord, or upper extremity nerves; burn victims; patients who are under anesthesia or sedated; critically ill patients; substance abusers or drug-overdosed patients; mentally ill or disabled patients; and infants and young children. Common causes of upper extremity compartment syndromes in these patients are prolonged limb compression due to drug overdose, extravasation of intravenous or arterial fluid administration, thrombolytic therapy for myocardial infarction, and trauma. If a compartment syndrome of the forearm, hand, or upper arm is suspected, the patient should be examined closely and frequently, and changes over time should be documented carefully. Adjunctive diagnostic techniques, particularly intracompartmental pressure measurement, play an essential role in these patients, in whom it may be difficult or impossible to assess symptoms and signs or make an accurate differential diagnosis by physical examination alone. Once a compartment syndrome is diagnosed, emergent fasciotomy is needed to prevent devastating loss of function.