Although it has been suggested that defective interleukin 7 receptor (IL-7R) signaling is one of the principal causes of severe combined immunodeficiency disease (SCID) in mice and humans, little is known about the molecular and clinical characteristics of human IL-7Ralpha mutations. We report a novel mutation of the IL-7Ralpha gene in a Korean SCID patient with a greatly diminished T-cell count but normal numbers of B-cells and natural killer (NK) cells. Using direct sequencing and restriction fragment length polymorphism analysis, we identified a C-->T nucleotide change at position 638. This change resulted in a nonsense mutation (R206stop) in this patient. Both parents were heterozygous for C/T at this site. The results of this study emphasize the importance of characterization of IL-7Ralpha mutations in SCID patients with diminished T-cell numbers but normal numbers of B-cells and NK cells.