Objective Written emotional disclosure for 15–20 min a day over 3 to 4 days improves physical and psychological health and may benefit cancer patients. However, no studies have tested the effectiveness of guided writing in cancer patients and their partners. A randomised controlled trial tested whether writing about the patient's diagnosis and treatment of ovarian cancer using the Guided Disclosure Protocol (GDP) is effective in reducing perceived stress and improving quality of life (QoL) in ovarian cancer couples. The study also tested two theories that may account for beneficial effects of written emotional disclosure, the cognitive processing hypothesis and the social interaction hypothesis. Methods Patients and their partners (N = 102 couples) were randomised to write at home for 15 min a day over 3 days about the patient's diagnosis and treatment using the GDP or what the patient did the previous day (control). Couples were assessed at baseline, 3‐ and 6‐month follow‐ups on the primary outcomes of perceived stress and QoL and secondary outcomes of intrusive thoughts (testing the cognitive processing hypothesis) and illness‐related couple communication (testing the social interaction hypothesis). Results There were no main effects for any outcomes. However, in patients, the GDP improved QoL if illness‐related couple communication improved and buffered the effect of intrusive thoughts on perceived stress. Conclusions The GDP might benefit patients in certain circumstances, through changes in communication (in line with the social interaction hypothesis). Further research is needed to determine whether patients benefit from interventions to improve illness‐related couple communication and under which conditions. Copyright © 2013 John Wiley & Sons, Ltd.