We have summarized the findings in selected congenital and acquired immunodeficiency disorders that frequently display blood and bone marrow abnormalities. Many other immunodeficiency syndromes were not discussed, as bone marrow abnormalities are infrequently seen in them. Examination of the bone marrow is not usually critical for diagnosing most immunodeficiency disorders. However, it may be diagnostic in the evaluation of fever of unknown origin, to rule out other causes of peripheral cytopenias, and to screen for virus-associated malignancies frequently seen in immune-deficient patients (for example, B cell lymphoma, Kaposi's sarcoma). Examination of the bone marrow is critical in the diagnosis of the virus-associated hemophagocytic syndrome and is often important in evaluating patients with the acquired immunodeficiency syndromes who display hematologic disturbances, such as those being treated with AZT, or those who might have opportunistic infections or malignant neoplasms.