Objective: Repetitive negative thinking (RNT) for example, worry in generalized anxiety disorder (GAD) and rumination in depression, is often targeted during psychological treatments. To test the hypothesis that negative interpretation bias contributes to worry and rumination, we assessed the effects of inducing more positive interpretations in reducing RNT. Method: Volunteers diagnosed with GAD (66) or depression (65) were randomly allocated to one of two versions of cognitive bias modification for interpretation (CBM-I), either with or without RNT priming prior to training, or a control condition, each involving 10 Internet-delivered sessions. Outcome measures of interpretation bias, a behavioral RNT task and self-reported worry, rumination, anxiety and depression were obtained at baseline, after home-based training and at 1-month follow-up (self-report questionnaires only). Results: CBM-I training, across diagnostic groups, promoted a more positive interpretation bias and led to reductions in worry, rumination, and depressive symptoms, which were maintained at follow-up. Anxiety symptoms were reduced only in the GAD group at follow-up. There were no differences between CBM-I versions; brief priming of RNT did not influence CBM-I effectiveness. Level of interpretation bias post training partially mediated the effects of CBM-I on follow-up questionnaire scores. Conclusions: In contrast to some recent failures to demonstrate improvements following Internet-delivered CBM, we found that self-reported RNT and negative mood were reduced by CBM-I. This is consistent with a causal role for negative interpretation bias in both worry and rumination, suggesting a useful role for CBM-I within treatments for anxiety and depression. What is the public health significance of this article? Many people worry about the future, or mull over negative events from the past (rumination). These types of unhelpful repetitive negative thinking can maintain clinical anxiety and depression. This study indicates that simple regular practice in making positive interpretations of emotionally ambiguous information reduces repetitive negative thinking in individuals with clinical anxiety or depression, and also improves mood.