Acute compartment syndrome of the lower leg is usually associated with major trauma such as fracture, and most commonly occurs in the anterior or deep posterior compartments. Isolated lateral compartment syndrome is less typical and usually associated with minor trauma or exertion. There are a few reports of the acute compartment syndrome which develops without an obvious cause. However, there are few reports of idiopathic acute isolated lateral compartment syndrome, and there are no reports of the radiologic findings. This study presents a radiologic finding of idiopathic acute isolated lateral compartment of lower leg, which shows localized enlargement of the peroneal muscles with peripheral convex bowing and change of their signal intensity with fluid signal along the adjacent fascial planes on magnetic resonance imaging.