This article discusses the various hematologic and oncologic diseases to consider when caring for a patient with HIV infection. These diseases are not only more common in this patient population, but they can often be more severe, leading to greater morbidity and mortality than would be expected for a patient without HIV infection. Among the hematologic conditions discussed are common blood dyscrasias such as anemia, leucopenia, and thrombocytopenia, as well as less common disease processes such as immune thrombocytopenic purpura, thrombotic thrombocytopenic purpura, and venous thromboses. The oncologic diseases discussed include AIDS-defining conditions, such as Kaposi sarcoma, invasive cervical carcinoma and non-Hodgkin lymphoma. The recognition of these conditions in patients infected with HIV is of paramount importance for identifying patients at high risk of morbidity and mortality.