Objective: The present research assessed the simultaneous effects of four attitude variables (cognitive attitudes, affective attitudes, anticipated negative affective reactions, and anticipated positive affective reactions) in the context of the theory of planned behavior (TPB) on blood-donation intentions and behavior. Methods: Experienced blood donors (N = 1108) completed questionnaires measuring attitude variables plus components of the TPB and a measure of attitudinal ambivalence in relation to giving blood again in the next six months. Records were used to assess whether participants subsequently donated blood again in the six months after completing the questionnaire. The main outcome measures were objectively assessed blood donation and intentions to make an additional donation of blood. Results: Confirmatory factor analysis supported a distinction between cognitive attitudes about giving blood, affective attitudes about giving blood, anticipated negative affective reactions about not giving blood, and anticipated positive affective reactions about giving blood. Multiple regression analyses indicated that perceived behavioral control, anticipated negative affective reactions, cognitive attitude, anticipated positive affective reactions and subjective norms were significant simultaneous predictors of intentions to donate blood. Logistic regression analyses indicated that intentions, perceived behavioral control, and anticipated positive affective reactions were significant, simultaneous predictors of blood donation. Attitudinal ambivalence significantly moderated the effects of cognitive attitudes on intentions, and the effects of anticipated negative affective reactions on both intentions and donation behavior. Conclusion: The findings point to the value of considering different types of attitudes, and anticipated negative affective reaction in particular, for predicting health behaviors. (PsycINFO Database Record (c) 2012 APA, all rights reserved).