Patients with idiopathic thrombocytopenic purpura (ITP), a disorder in which antibody coated platelets are cleared from the circulation by phagocytic cells, are often treated with glucocorticoids. The effect of corticosteroids on the recognition and ingestion of sensitized platelets by phagocytes can be quantified in these patients and compared to changes in platelet levels. Six patients with ITP were treated with 96 mg daily of methylprednisolone for 5 days. This treatment raised their platelet count and simultaneously decreased the ability of their granulocytes to phagocytize antibody-coated platelets and C3-coated paraffin oil droplets. Corticosteroid treatment did not affect the binding of antibody to platelets or the quantity of antibody in the patients' serum. The ingestion defect was present in isolated, washed leukocytes and persisted for 3–5 days after the corticosteroids were discontinued. Granulocytes and purified monocytes obtained from patients with other medical disorders receiving corticosteroids also ingested paraffin oil droplets and opsonized platelets at a slower rate. These studies provide direct evidence that corticosteroids induce a generalized phagocytic defect and that this may be the mechanism by which corticosteroids raise the platelet count in patients with ITP.