Atraumatic limb pain and limp is a common pediatric presentation in the emergency department in the United States. In a majority of cases, these presentations are benign. However, in cases where pediatric patients are repeatedly presenting for atraumatic limb pain, further investigation is required. We present such the case of a 14-year-old female with acute worsening of progressive atraumatic knee pain who was found to have a Brodie's abscess, a subacute pyogenic form of osteomyelitis. This is a particularly challenging diagnosis, as it often presents with no associated symptoms such as fever or weight loss. The consequences of missing this diagnosis include permanent disability and potential amputation, but excellent outcomes can be expected for those who undergo timely surgical debridement and irrigation. We discuss the etiology, common presentations, and treatment of this rare but potentially limb-threatening disease in the hope that clinicians will consider this diagnosis in cases of persistent or progressive atraumatic limb pain.