Three experiments assessed the effects of mood on symptom appraisal, health behavior self-efficacy, outcome expectations, and perceptions of vulnerability. Ss in Experiments 1 and 2 were acutely ill, whereas Ss in Experiment 3 were healthy. In each experiment, happy, sad, and neutral moods were induced. In Experiment 1, Ss who experienced sadness reported more aches and pains and greater discomfort than happy Ss. Sad Ss were less confident that they could carry out illness-alleviating behaviors. Experiments 2 and 3 demonstrated that mood's influence on vulnerability perceptions is moderated by health status. Although mood had little impact on perceptions of vulnerability among ill Ss, probability estimates of future negative health-relevant events among healthy Ss were mood sensitive. Seeing oneself as invulnerable to future negative events was accentuated among happy Ss and attenuated among sad Ss. Mood may be an important determinant of care seeking, adherence, and recovery from illness.