Gemtuzumab ozogamicin is a humanised monoclonal IgG4 antibody, linked to a cytotoxic calicheamicin derivative. It effects cell necrosis by specifically targeting the CD33 antigen which is expressed on the surface of leukaemic cell blasts in more than 90% of patients with acute myeloid leukaemia (AML), but is not present on normal stem cells. Therapy with gemtuzumab ozogamicin (2 doses of 9 mg/m2) in 3 noncomparative studies produced complete remission in 16% of adult patients with AML in first relapse, and complete remission with incomplete platelet recovery in an additional 13% of patients. Rates of remission did not differ between those aged less than 60 years and older than 60 years. Many patients were able to receive both doses of gemtuzumab ozogamicin therapy as outpatients. Survival duration was similar between those treated as outpatients and those requiring hospitalisation. About one-third of 11 children and adolescents treated with 2 doses of 9 mg/m2 gemtuzumab ozogamicin in a phase I study showed <5% bone marrow blasts after completion of therapy. The most commonly encountered adverse events in clinical trials with gemtuzumab ozogamicin were myelosuppression, increased levels of hepatic enzymes, infection, fever, bleeding, chills, nausea and vomiting and dyspnoea. No treatment-related renal failure or alopecia was reported.