Thrombocytopenia may be a benign, incidental finding in an asymptomatic patient or the sign of a potentially life-threatening disorder. The history, physical examination and peripheral blood smear can assist the physician in determining the diagnosis and treatment. After the initial blood count is repeated to help eliminate the possibility of laboratory error, a thorough history, complete physical examination, complete blood cell count and appropriate laboratory tests are required. The history may reveal related illnesses, risk factors such as infection or drug use, or a family history suggestive of congenital thrombocytopenia. The physical examination should concentrate on the lymphatic and hepatosplenic systems, with the physician looking for jaundice, fever or petechiae. With the review of a complete blood cell count and a peripheral smear examination, the initial work-up is completed and may prevent additional, unnecessary testing. Etiology-specific tests follow if needed. Serious spontaneous bleeding is usually a risk only in patients with platelet levels under 20,000 per mm3.