Background: Attachment anxiety and avoidance have been found to be associated with worry. A substantial body of research suggests that intolerance of uncertainty (IOU) is a phenomenon which predicts level of worry, yet the relationship between attachment, IOU and worry remains to be investigated. Methods: The present study recruited 281 participants from the community within Australia to complete an online survey incorporating the Intolerance of Uncertainty Scale, the Experiences in Close Relationships Scale-Revised, and the Penn State Worry Questionnaire. Results: The analyses revealed attachment anxiety and attachment avoidance were positively correlated with IOU, and that attachment anxiety, attachment avoidance and IOU were positively correlated with worry. Furthermore, IOU was found to mediate the relationship between attachment anxiety and worry, even when attachment avoidance was entered as a covariate. By contrast, for the relationship between attachment avoidance and worry, intolerance of uncertainty only mediated the association if attachment anxiety was ignored. Conclusions: The results of the study suggest that IOU may play a key role in the relationship between adult attachment difficulties and the experience of worry. These findings may have significant implications for the conceptualisation and psychological treatment of attachment related difficulties. (C) 2017 Elsevier Ltd. All rights reserved.