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Background:  Risk factors, virologic parameters and outcomes associated with HHV-6 viremia in high-risk donor CMV-seropositive and recipient CMV-seronegative (D+R-) liver transplant recipients in the current era are incompletely defined. Methods:  The study population consisted of patients in the preemptive therapy (PET) arm of a randomized, controlled trial of PET versus valganciclovir prophylaxis for CMV prevention in D+R- liver transplant recipients. Weekly blood samples through 100 days in the PET group were tested for HHV-6 viremia using a real-time quantitative PCR. Assessments included virologic characteristics and relationship with CMV, risk factors and impact of HHV-6 viremia with outcomes through 12 months post-transplant. Results:  HHV-6 viremia at any level developed in 42% (40/96). Older patient age (p=0.03), longer hospitalization (p=0.015), and ICU stay at transplantation (p=0.029) were significantly associated with high-grade viremia. Concurrent HHV-6 and CMV viremia was associated with earlier onset of HHV-6 viremia (p=0.004), higher HHV-6 AUC (p=0.043), and higher peak HHV-6 viral load (p=0.006) vs HHV-6 viremia alone. High-grade viremia was independently associated with biopsy-proven rejection within 12 months (p=0.045) post-transplant. Conclusions:  Among D+R- liver transplant recipients receiving valganciclovir as PET, high-grade HHV-6 viremia was associated with increased age and critical-illness in ICU at time of transplant and was independently associated with allograft rejection.