There is evidence from animal studies that 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) impairs immune responses, with the thymus being a principal target organ. The purpose of this study was to evaluate thymic function, through measurement of thymic hormone levels, in persons exposed to TCDD. We examined thymosinalpha‐1(Thya‐1) levels in sera from a group of 94 persons who were presumed to be exposed to TCDD from living, working, or recreating in a contaminated residential area. We compared these results, along with results from in vitro and in vivo tests of immune function, with those from a group of 105 unexposed persons who were similar with regard to age, sex, and race. The exposed group had a significantly lower mean Thya‐1serum level (977.3 ± 304.1 pg/ml vs. 1148.7 ± 482.1 pg/ml,p< .01 by t‐test). We also found a statistically significant trend of decreasing Thya‐1levels with increasing number of years of residence in the TCDD‐contaminated area. However, Thya‐1levels were not associated with other measures of immune function in the TCDD‐exposed group. Thus, while the principal findings suggest that long‐term TCDD exposure may be associated with diminished secretion of Thya‐1. the lack of an association with an increased prevalence of clinically diagnosed immune suppression in these TCDD‐exposed persons makes the biologic significance of the findings unclear. Further studies are needed to more fully evaluate possible long‐term TCDD‐induced effects on the thymus and human immune function.