Male ornaments may advertise genetic benefits to females choosing mates. These benefits may come in the form of genes for resistance to parasites and disease. Thus, females that prefer more ornamented males as mates may receive genes for enhanced immune system function for their offspring. The common yellowthroat (Geothlypis trichas) is the only species to date in which a male plumage ornament (size of the black facial mask) is known to be related to extrapair mating, and extrapair young are more immunocompetent than their within-pair half-sibs (at least in colder years). To investigate whether male mask size signals superior immune function, we examined male ornaments (mask and bib size and color) in relation to measures of overall health (hematocrit) and immune system function (plasma immunoglobulin G [IgG] concentration and cutaneous immune activity). We also investigated the role that testosterone may play in mediating the relationship between ornaments and immunity. Male mask size was correlated positively with IgG level, suggesting that male mask size may indicate humoral immunity. However, mask size was correlated negatively with hematocrit and cutaneous immune activity (our measure of nestling immunocompetence). Bib size and color were not related to these indices of immune function or health. Plasma testosterone level was neither related to immune function or health nor to the expression of male ornaments. These results suggest that there might be a trade-off between immune system components, as well as between immunity and the production or advertisement of male ornaments.