Because routine blood testing is so common in modern medicine, many cancers are first detected by the discovery of a blood test abnormality. Cancer may indirectly affect both the cellular elements of the blood as well as the coagulation system. This review of the more common hematological syndromes associated with neoplasia provides an overview of the more common entities found in clinical practice. These syndromes are important to recognize for several reasons. They provide insight into disease mechanisms and may allow earlier detection of cancer. Even if the cancer is resistant to antitumor therapy, treatment of the hematologic disorder may provide significant symptom relief.