Graft-versus-host disease is an uncommon complication of solid-organ transplant and is associated with a high rate of mortality. Here, we describe a female patient with primary biliary cholangitis who developed graft-versus-host disease following an orthotopic liver and renal allotransplant from a deceased male donor. Systemic donor lymphoid chimerism is one of several important findings to confirm a diagnosis of graft-versus-host disease after solid-organ transplant, along with clinical and histologic findings. In this case, cytogenetic analyses and chimerism studies performed on blood, blood components, and bone marrow specimens obtained at several timepoints supported the diagnosis of graft-versus-host disease and demonstrated sustained near-complete donor engraftment of the lymphoid compartment. This case report illustrates the utility of chimerism testing to rapidly diagnose this serious condition in patients who have received a solid-organ transplant.