We administered gemtuzumab ozogamycin ("mylotarg"; 9 mg/m(2) day 1 or 5) and all-trans retinoic acid (ATRA) to 19 patients with untreated acute promyelocytic leukemia (APL). There were 3 patients who also received idarubicin because of a white blood cell (WBC) count of more than 30 000/microL. In complete remission (CR), patients were to receive 8 courses of mylotarg (9 mg/m(2) every 4 to 5 weeks) and ATRA; idarubicin was added only for persistent or recurrent polymerase chain reaction (PCR) positivity. The CR rate was 16/19 (84%). All 12 patients tested to date were PCR-negative 2 to 4 months from CR date; none of the 7 patients evaluated subsequently have reverted to PCR positivity (median follow-up in CR was 5 months, up to 14 months). Mylotarg was well tolerated. A median of 5 post-CR courses have been given to date with 3 patients having currently received 8 post-CR courses, and 4 patients receiving 7 post-CR courses. Mylotarg appears active in APL, and repeated administration is feasible.