Objective: The state-trait nature of alexithymia is not fully resolved. The aim of this article was to evaluate the temporal stability of alexithymia in cancer patients after psychological intervention. Design: Absolute and relative stability of alexithymia was evaluated in 104 cancer patients who received multicomponent psychological treatment or usual medical care. Results: Alexithymia scores significantly decreased in treated patients and increased in the control group, thus showing that the lack of absolute stability (i.e., the change of personality scores over time) likely was because of the effect of the treatment versus the control condition. The relative stability (i.e., the extent to which the inter-individual differences remain the same over time) of alexithymia after 6 months was, however, demonstrated with multiple regressions, after controlling for anxiety, depression, and cancer-related pain. Conclusions: This study confirms the relative stability of alexithymia and that the lack of absolute stability of the construct is influenced by psychological intervention in cancer patients. © 2011 Wiley Periodicals, Inc. J Clin Psychol 67:1–11, 2011.