Acute-on-chronic exertional compartment syndrome is a rare and severe progression of the likely common and more benign chronic exertional compartment syndrome. This is a report of one 17-year-old male on a pediatric inpatient service with bilateral anterior leg pain of unknown origin. Because of the nonspecific nature of pain, a high level of suspicion is required for timely diagnosis to avoid compartment ischemia and irreversible soft tissue and nerve damage. While high-energy orthopaedic trauma, orthopaedic surgery, or closed reduction and casting are common preceding events for compartment syndrome, this patient presented with acute-on-chronic exertional compartment syndrome. A dearth of literature of this condition hampered its morbiditysparing diagnosis. While there is a spectrum of clinical findings for the acute decompensation of chronic exertional compartment syndrome, like any compartment syndrome, pain disproportionate to physical exam is the most sensitive sign. Understanding the exertional compartment syndrome spectrum is tantamount to avoid the devastating complications of a missed diagnosis of acute compartment syndrome.