The outcomes of treatment with gemtuzumab ozogamicin compared with those of conventional chemotherapy for relapse of acute myelogenous leukemia (AML) were studied. The gemtuzumab ozogamicin group consisted of 104 patients treated in 15 U.S. and 25 European centers for first relapse of AML between May 1997 and December 1998. Conventional chemotherapy recipients consisted of 22 historical control patients who received treatment for their first occurrence of AML at a Boston teaching hospital between January 1991 and December 1994 and who subsequently underwent conventional inpatient reinduction chemotherapy for relapse. Patients in the gemtuzumab ozogamicin group received a two-hour i.v. infusion of 9 mg/m2 for up to three doses with at least two weeks between doses, while the historical controls received conventional chemotherapy, usually consisting of continuous-infusion cytarabine (days 1-7) plus mitoxantrone. Outcomes evaluated included differences in survival and hospitalization (occurrence and duration) for up to six weeks. Both stratified and multivariate regression methods were used in making comparisons. Adjusting for baseline differences, six-week survival rates were similar in the two groups (95% for the historical control group versus 89% for the gemtuzumab ozogamicin group). However, gemtuzumab ozogamicin recipients had significantly fewer total hospital days (adjusted mean, 19 days, versus 35 days), which was consistent with the higher prevalence of outpatient chemotherapy in this group (43% versus 0% among the historical controls). The difference in the number of hospital days was consistent across various baseline and demographic factors. Gemtuzumab ozogamicin appeared to be associated with equivalent survival and fewer total days of hospitalization than conventional chemotherapy in adults with relapsed AML.