Background: Nightmares are extremely dysphoric dreams, which are prevalent and associated with psychological strain. This study investigated (a) the efficacy of an internet-based imagery rehearsal therapy (IRT), (b) the role of imagery rescription, and (c) the role of guidance during internet-based IRT. Methods: A total of 127 patients suffering from mainly idiopathic nightmares were randomly assigned to 1 of 2 IRT internet-based groups (guided IRT; unguided IRT) or to 1 of 2 active control groups (frequency control group; narrative control group). Results: IRT was more effective than a nightmare frequency control condition with respect to nightmare frequency and nightmare distress. Compared to the narrative control group, IRT was only superior in improving nightmare distress but not in nightmare frequency because the narrative control group also improved regarding nightmare frequency. Guidance by a nightmare coach did not affect efficacy, compliance, or dropout. Conclusion: Internet-based IRT seems to be an effective treatment even when offered with minimal guidance by a nightmare coach. Describing the nightmare narrative in detail already decreased nightmare frequency. However, with regard to inducing decreases in nightmare frequency and nightmare distress, IRT was superior to the narrative control group. The results are discussed with reference to the mastery hypothesis.