Suppression of various functions of T cells derived from cancer patients has been linked previously to changes in the T-cell receptor (TCR)-associated signal transduction molecules, in particular the ζ chain of the TCR complex. In this study, we have examined the TCRζ chain expression and cytokine production in vivo and in vitro in T cells of patients with metastatic adenocarcinomas of the pancreas that participated in a Phase I clinical trial of the MUC1 peptide plus bacillus Calmette-Guérin cancer vaccine. A majority of the patients had reduced TCRζ chain expression and interleukin 4 production by T cells, and all of the patients showed decreased production of IFN-γ of their peripheral T cells when compared with healthy individuals. Peripheral blood T cells were activated with the phorbol ester phorbol myrisate acetate and ionomycin to show that although aberrant TCRζ chain expression and decreased cytokine production were often correlated, the reduced cytokine production was not simply a consequence of an impaired TCRζ chain expression. Rather, these are two separate but parallel defects in signal transduction in T cells, which are potentially modulated by the same mechanisms. Half of the patients showed an improvement for TCRζ chain or IFN-γ expression after vaccination.