Thrombocytopenia is frequently encountered in patients with solid tumors or hematologic malignancies. In these patients, it may be related to the underlying disease or it may be a consequence of treatment. In some cases, thrombocytopenia may be an isolated hematologic defect. However, thrombocytopenia in cancer patients will often be accompanied by variable amounts of anemia and/or neutropenia. In the majority of cases, thrombocytopenia in cancer patients is not clinically significant, but is discovered during routine blood counts. However, some of the causes of thrombocytopenia indicate the development of serious disorders which should be addressed promptly. Since the etiology of thrombocytopenia in cancer patients may be multifactorial, diagnosis is often complex. The main causes of thrombocytopenia are increased platelet destruction and decreased platelet production. Less frequent are splenic sequestration and dilutional thrombocytopenia.