One hundred and fifty children with AML in first remission were treated with allogeneic BMT in two sequential studies of the Childrens Cancer Group. The absence of differences in baseline variables justified comparison between the two studies. In the initial study (CCG-251), patients received GVDH prophylaxis with MTX alone (17 doses over 102 days). In an attempt to diminish the morbidity and mortality of acute GVDH, a second study (CCG-213) employed stronger GVHD prophylaxis with 6 months of CYA and short-course MTX (four doses over 11 days). Outcome was compared between these two non-randomized populations of children with AML transplanted in first remission. Augmented GVHD prophylaxis substantially diminished treatment-related mortality from 31% to 11% (p = 0.0033), but this effect was counterbalanced by an increase in the relapse risk from 22% to 35% (p = 0.29). Event-free survival at 2 years was 54% on CCG-251 and 59% on CCG-213 (p = 0.21). We observed a marginal diminution of relapse risk among patients with chronic GVHD compared with those without chronic GVHD (19% vs. 35%, respectively; p = 0.10). No anti-leukemic effect of acute GVHD was observed.