Zidovudine may cause bone marrow toxicity, most commonly neutropenia and anemia, particularly in patients with advanced symptomatic HIV disease. Hematologic toxicities appear to be associated with bone marrow reserve prior to therapy and to dose and duration of treatment. Zidovudine should be used with extreme caution in patients with bone marrow suppression indicated by a granulocyte count below 1000 cells/mm3 or hemoglobin less than 9.5 g/dL. Routine blood counts are recommended, and generally should occur more frequently in patients with advanced disease. If bone marrow toxicity occurs, a dose adjustment, dosage interruption, or discontinuation of zidovudine therapy and/or blood transfusions may be necessary.