Leukocytosis, a common laboratory finding, is most often due to relatively benign conditions (infections or inflammatory processes). Much less common but more serious causes include primary bone marrow disorders. The normal reaction of bone marrow to infection or inflammation leads to an increase in the number of white blood cells, predominantly polymorphonuclear leukocytes and less mature cell forms (the "left shift"). Physical stress (e.g., from seizures, anesthesia or overexertion) and emotional stress can also elevate white blood cell counts. Medications commonly associated with leukocytosis include corticosteroids, lithium and beta agonists. Increased eosinophil or basophil counts, resulting from a variety of infections, allergic reactions and other causes, can lead to leukocytosis in some patients. Primary bone marrow disorders should be suspected in patients who present with extremely elevated white blood cell counts or concurrent abnormalities in red blood cell or platelet counts. Weight loss, bleeding or bruising, liver, spleen or lymph node enlargement, and immunosuppression also increase suspicion for a marrow disorder. The most common bone marrow disorders can be grouped into acute leukemias, chronic leukemias and myeloproliferative disorders. Patients with an acute leukemia are more likely to be ill at presentation, whereas those with a chronic leukemia are often diagnosed incidentally because of abnormal blood cell counts. White blood cell counts above 100,000 per mm3 (100 x 10(9) per L) represent a medical emergency because of the risk of brain infarction and hemorrhage.