Our group has recently described a gene on human chromosome 21, the Cold Sore Susceptibility Gene-1 (CSSG-1, also known as C21orf91), which may confer susceptibility to frequent cold sores in humans. We present here a genotype-phenotype analysis of CSSG-1 in a new, unrelated human population. Seven hundred fifty-eight human subjects were enrolled in a case/control Cold Sore Study. CSSG-1 genotyping, herpes simplex virus 1 (HSV1) serotyping, demographic and phenotypic data was available from 622 analyzed subjects. Six major alleles (H1-H6) were tested for associations with each of the self-reported phenotypes. The statistical analysis was adjusted for age, sex and ethnicity. Genotype-phenotype associations were analyzed from 388 HSV1-seropositive subjects. There were significant CSSG-1 haplotype effects on annual cold sore outbreaks (P=0.006), lifetime cold sores (P=0.012) and perceived cold sore severity (P=0.012). There were relatively consistent trends toward protection from frequent and severe cold sores among those with the H3 or H5/6 haplotypes, whereas those with H1, H2, and H4 haplotypes tended to have more frequent and more severe episodes. Different alleles of the newly described gene CSSG-1 affect the expression of cold sore phenotypes in this new, unrelated human population, confirming the findings of the previous family-based study.