Compartment syndrome is a limb-threatening and occasionally life-threatening injury. It occurs whenever the tissue pressure (referred to as interstitial pressure) within a closed anatomic space is greater than the perfusion pressure. Untreated, compartment syndrome leads to tissue necrosis, permanent functional impairment and, if severe in large compartments, renal failure and death. Compartment syndrome can occur within any muscle group located in a compartment. It is most common following an event that severely damages a muscle, like a crushing or twisting injury. Mechanisms of injury that involve circumferential burns, ischemia and tourniquets can cause compartment syndrome. Motorcyclists who suffer lower-extermity injuries in accidents are a high-risk group. A tough membrane referred to as a fascia covers muscle groups, forming a compartment for the muscle. In normal circumstances, this arrangement allows the muscle to function more efficiently, but if the muscle is injured in any way, the fascia limits the amount of swelling that can occur. This in turn restricts the flow of blood through the affected region. The first compromised function within the compartment is the flow of lymph and venous blood. If there are sensory nerves running through the compartment, they will not function correctly, causing the numbness, tingling and, later, the pain associated with compartment syndrome. With more swelling, arterial flow is compromised, pain worsens and motor function is impaired. An artificial way of producing a compartment syndrome is to place a cast or splint around a damaged extermity, compressing it. This is a way emergency personnel can compromise an injury and cause long-term consequences for the patient. Recovery is achieved by surgically opening the compartment involved (a fasciotomy) and releasing the pressure. The muscle at first will swell outside the compartment, but then it recovers, swelling is reduced and normal function can be recovered. Prehospital treatment of extremity injuries that will prevent or limit compartment syndrome is immobilization, elevation and cooling. Recognition of the syndrome later in its course, as in this case, requires the EMT to remove the patient to an appropriate emergency department. Prehospital providers need to recognize that many mechanisms of injury can produce this syndrome, even those that seem relatively minor. All injured patients should be educated to seek care should the symptoms of numbness, deep pain and coolness to the distal extremity occur. This case involved a patient who, from a relatively minor mechanism of trauma, experienced an internal disruption of the muscle group controlling the thumb (thenar mass). The early swelling in the thenar compartment resulted in the patient experiencing a tingling sensation in his left thumb. In many cases, such an injury would be referred to as a "stinger" (a temporary neurological deficit due to a sudden and excessive stimulation of a neurologic plexus or junction). But this patient had more swelling in the compartment, resulting in a lack of circulation manifested by a cool extremity, poor capillary refill and decreased pulse oximetry. Luckily, this officer recognized the need for medical evaluation of what appeared to be a minor injury and was returned to duty with no permanent impairment.