Leukostasis (also called symptomatic hyperleukocytosis) is a medical emergency most commonly seen in patients with acute myeloid leukemia. It is characterized by an extremely elevated blast cell count and symptoms of decreased tissue perfusion. The pathophysiology of leukostasis is not well understood, but inadequate delivery of oxygen to the body's cells is the result. Leukostasis is diagnosed when white cell plugs are seen in the microvasculature. The most common symptoms are dyspnea and hypoxia, usually accompanied by visual changes, headaches, dizziness, confusion, somnolence, and coma. Prompt treatment is required since, if left untreated, it has a very high mortality rate. Treatments aim to rapidly reduce white blood cell counts while also treating the underlying disorder.

Symptomatic hyperleukocytosis (leukostasis) is defined by a high blast cell count^[quantify] along with symptoms of decreased tissue perfusion. Leukostasis is associated with people who have bone and blood disorders and is very common among people with acute and chronic myeloid leukemia. Leukostasis is a pathologic diagnosis that inhibits efficient flow to the microvasculature of the body. Continued and untreated leukostasis presents respiratory and neurological distress simultaneously and is a medical emergency, with mortality rates reaching between 20 and 40 percent when untreated. A leukemia blood cell count greater than 50×10⁹/L or 100×10⁹/L signifies hyperleukocytosis. Above 100×10⁹/L, symptoms of leukostasis start.

Individuals affected by leukostasis may present with respiratory symptoms such as cough, difficulty breathing, breathing too quickly, or inadequate levels of oxygen in the blood requiring support with a mechanical ventilator. Neurologic symptoms, such as temporary confusion, blurry vision, dizziness, ringing in the ears, ataxia, stupor, sleepiness, headaches, and coma, may be seen. Neurologic signs such as seizures, focal neurologic deficits (e.g., weakness in one arm or leg), swelling of the retina, retinal bleeding, and dilated blood vessels on inspection of the back of the eye. Rare complications of leukostasis include renal vein thrombosis, priapism, and acute ischemia of the leg.

The most common symptom is fever, which is often linked with inflammation and possible infection. Less common symptoms include myocardial ischemia or right ventricular overload, increased acute kidney injury, priapism, acute limb ischemia and bowel infarction.

In symptomatic leukocytosis caused by leukemia, it is common to find leukostasis in all their organs. The majority of the time, patients die from neurological complications (roughly 40%) instead of particular organ damage. The lungs alone account for approximately 30 percent of the deaths. All other organs combined attribute to 30 percent, with the major outliers being neurological and respiratory failure equating to 70 percent. Damage to the microvasculature of the body is the primary cause of death. Microvasculature damage to the lungs is second only to neurological damage because the body is already experiencing hypoxic conditions, which leads to lung tissue damage.

Hyperleukocytosis is very common in acutely ill patients. It occurs in response to a wide variety of conditions, including viral, bacterial, fungal, or parasitic infections, cancers, hemorrhages, and exposure to certain medications.

For lung diseases such as pneumonia and tuberculosis, where leukocytosis is usually present, white blood cell count can aid diagnosis.

Specific medications, including corticosteroids, lithium and beta agonists can cause hyperleukocytosis.