Gemtuzumab is a cancer medication. Gemtuzumab interferes with the growth of cancer cells and slows their growth and spread in the body. Gemtuzumab is used to treat acute myeloid leukemia, a type of blood cancer. Gemtuzumab is usually given to people who are at least 60 years old and have a relapse of their disease and who cannot receive other cancer medications. Gemtuzumab may also be used for other purposes not listed in this medication guide.