Eleven patients with inherited hemoglobinopathies, red cell dyscrasias, or coagulopathies underwent open heart surgery with no deaths and no evident complications resulting from the hematologic defects. Analysis of the various defects led to recommendation of precautions appropriate to each condition. Thalassemia minor and elliptocytosis require few precautions. Sickle cell trait should be tested for in black patients and hypoxia and acidosis carefully avoided. Glucose-6-phosphate dehydrogenase deficiency should be tested for in blacks, and drugs that precipitate hemolytic crises avoided. In hereditary spherocytosis, splenectomy is to be recommended before operation and prosthetic valves avoided. For von Willebrand disease, careful testing of patients with a history of bleeding is recommended, cryoprecipitates given when the diagnosis is made, and certain drugs avoided.