Lactic acidosis in the emergency department and other hospital settings is typically due to tissue hypoxia with sepsis being the most common cause. However, in patients with persistently elevated lactate without evidence of inadequate oxygen delivery, type B lactic acidosis should be considered. We report the case of a 12-year-old boy with relapsed and refractory pre–B-cell acute lymphoblastic leukemia who presented in distress with tachycardia, history of fever, and diffuse abdominal tenderness. The patient had severe metabolic acidosis with elevated lactate upon arrival to the emergency department. Despite aggressive fluid resuscitation and intravenous antibiotics, the patient's acidosis worsened. Serial blood cultures were negative, and he was eventually diagnosed with type B lactic acidosis secondary to relapsed acute lymphoblastic leukemia.