Microbiomes of immunocompromised hosts differ from those of persons with intact immune function. The impact of microbiome on well-being and disease among immunocompromised hosts is poorly understood, and it can only be defined by studying carefully-chosen patient populations. The Immunocompromised Hosts Program is dedicated to basic, translational and clinical research on the relationships between the microbiome and clinical conditions in diverse populations with suppressed immune function. The Program will make use of the unparalleled resources of the University of Pittsburgh and University of Pittsburgh Medical Center, which are international leaders in solid organ transplantation, oncology, immunology and the treatment of immunosuppressed patients. Insights from research within the Program will inform the development of novel therapeutic, preventive and diagnostic strategies. Research activities will overlap and synergize with those of the Center’s ICU Microbiome and Antibiotic Resistance Programs.