This study is designed to determine the safety, tolerance and efficacy of POZ used as prophylaxis of IFI in high-risk subjects with Grade II-IV acute graft vs. host disease (GVHD) or extensive chronic GVHD. The primary objective is to assess the efficacy of SCH56592 vs fluconazole in preventing proven or probable IFI within the time period from randomization to 16 weeks after the start of treatment with study drug.