One objective of the present research was to examine the immunological effects of self-disclosing personal information regarding a traumatic or stressful experience. A second objective was to examine the hypothesis that the effect of self-disclosure on immune function is moderated by individual differences in cynical hostility. Forty-three male college undergraduates, classified as high or low on the Cook-Medley Hostility scale were randomly assigned to either a verbal self-disclosure or a nondisclosure discussion condition. Task-induced change in natural killer (NK) cell activity (i.e., cytotoxicity) served as the dependent variable. As predicted, a significant interaction between discussion condition and hostility was obtained. Among subjects in the self-disclosure condition, high hostility subjects exhibited a significantly greater increase in NK cell cytotoxicity than low hostility subjects. The effect of self-disclosure on NK cell activity is moderated by an individual's level of cynical hostility. The greater short term enhancement in NK cell activity observed for hostile persons is a likely correlate of a more pronounced acute arousal response elicited by the self-disclosure task.