ABSTRACT Employees are frequently interrupted when doing their work, sometimes with tasks that seem unreasonable or unnecessary. Drawing on stress-as-offence-to-self and stress appraisal theories, we expected that perceived illegitimacy of an interrupting task is detrimental to employees’ affective (i.e., anxiety, fatigue, vitality) and cognitive (i.e., task focus, attention residue) reactions to an interruption, because such interruptions are appraised as more of a hindrance and less of a challenge. To test this, we first used a within-persons recall study, where participants (N = 144) recalled two interrupting tasks, one perceived as legitimate, and one illegitimate. As expected, perceived illegitimacy of the interrupting task was associated with more anxiety and less task focus, because of more hindrance appraisal. Perceived illegitimacy also was associated with less vitality, task focus, and more attention residue, because of less challenge appraisal. Second, using a between-persons manipulation of an interrupting task in a work simulation (N = 231), perceived illegitimacy was associated with more anxiety, fatigue, attention residue, and less task focus, because of more hindrance appraisal. Furthermore, participants performed worse on the interrupting task because of more hindrance appraisal and less task focus. Overall, interrupting tasks perceived as illegitimate can be more negatively impacting, because such interruptions are appraised as hindering.