Invasive fungal infections (IFIs) are a leading cause of infection-related mortality in patients with acute leukemia and prolonged neutropenia and in allogeneic hematopoietic stem cell transplant (HSCT) recipients with graft-versus-host disease (GVHD). Although invasive candidiasis was the principal IFI predating fluconazole prophylaxis, invasive aspergillosis and other mold infections now cause most deaths from fungal infection in this patient population. The availability of broad-spectrum antifungal agents that can be safely administered over prolonged periods has stimulated interest in using mold-active prophylactic agents early as prophylaxis rather than later as therapy for suspected or documented IFIs. Two recent, prospective, randomized trials have shown a clear benefit of posaconazole prophylaxis in patients with myelodysplastic syndrome and acute mye-logenous leukemia with prolonged neutropenia and in allogeneic HSCT recipients with severe GVHD. In contrast, the peer-reviewed published database on the strategy of preemptive antifungal therapy, in which yeast-active prophylaxis (fluconazole) or no antifungal prophylaxis is used initially and modifications are triggered by a combination of laboratory markers and chest CT scans, is currently limited to an open-label feasibility study. Does sufficient evidence currently exist that the net benefit of the preemptive approach is at least on a par with posaconazole prophylaxis in the specific patient groups that were studied? The authors believe not and that more research is needed before the pre-emptive strategy can be recommended.