Next-generation sequencing (NGS) of immune receptors has become a standard tool to assess minimal residual disease (MRD) in patients treated for lymphoid malignancy, and it is being used to study the T-cell repertoire in many clinical settings. To better understanding the potential clinical utility and limitations of this application outside of MRD, we developed a BIOMED-2 primerebased NGS method and characterized its performance in controls and patients with graft-versus-host disease (GVHD) after allogeneic hematopoietic transplant. For controls and patients with GVHD, replicate sequencing of the same T-cell receptor b (TRB) libraries was highly reproducible. Higher variability was observed in sequencing of different TRB libraries made from the same DNA stock. Variability was increased in patients with GVHD compared with controls; patients with GVHD also had lower diversity than controls. In the T-cell repertoire of a healthy person, approximately 99.6% of the CDR3 clones were in low abundance, with frequency <10À3 . A single library could identify >93% of the clones with frequency !10 À3 in the repertoire. Sequencing in duplicate increased the average detection rate to >97%. This work demonstrates that NGS reliably and robustly characterizes TRB populations in healthy individuals and patients with GVHD with frequency !10 À3 and provides a methodologic framework for applying NGS immune repertoire methods to clinical testing applications beyond MRD. (J Mol Diagn 2017, 19: 72e83; http://dx.doi.org/10.1016/j.jmoldx.2016 Study and analysis of T-cell receptor (TCR) repertoires are essential for better understanding the development and reconstitution of immune systems, for providing insights into the pathogenesis of immune disorders, and for diagnosing and assessing therapeutic interventions for diseases driven by T cells. Next-generation sequencing (NGS) technology has become a standard method for assessing minimal residual disease (MRD) in patients with lymphoid malignancies. From a technical standpoint, MRD is a relatively simple application of NGS technology, essentially looking for known malignant or related somatically mutated clones; by combining replicate libraries, a sensitivity of 1 in 10 À5 has been demonstrated for MRD. Beyond detecting MRD, NGS is also an innovative and promising approach to assess clonal populations of T cells in healthy people during immune system development, patients with suspected malignancy, response to targeted therapies, immune reconstitution in patients undergoing hematopoietic cell transplant, and autoimmune diseases. 7,9e25 However, assessment of the T-cell repertoire in individuals is a far more complex task than assessment of MRD. In consideration of extending NGS methods beyond MRD applications in clinical laboratories, we examined the opportunity and technical limitations of the widely used BIOMED-2 primer sets and the MiSeq system (Illumina Inc., San Diego, CA) as an NGS application to assess the TCR repertoire. We evaluated replicate testing of individual libraries (th...