BACKGROUND: There is evidence that both breast cancer patients and their partners are affected emotionally, when facing a breast cancer diagnosis. Several couple interventions have been evaluated, but there is a need for couple intervention studies with a clear theoretical basis and a strong design. The Hand in Hand intervention is designed to enhance interdependent coping in the couples and to address patients and partners that are both initially distressed and non-distressed. METHODS: The Hand in Hand study is a randomised controlled trial among 199 breast cancer patients and their partners. Couples were randomised to 4-8 couple sessions with a psychologist in addition to usual care, or to usual care only, approximately 2 months after the patients' primary surgery date. The intervention was delivered within 3 months, and outcomes were assessed prior to randomisation and 5 and 10 months after primary surgery date. The primary outcome is patients' cancer-specific distress at the 5-month follow-up measured by the Impact of Event Scale. Secondary outcomes are assessed for both breast cancer patients and partners. These outcomes are: general distress, symptoms of anxiety and depression, health-related quality of life and measures of dyadic adjustment, intimacy and partner involvement. Cancer-specific distress is also assessed for partners. Eligible patients were women ≥ 18 years newly diagnosed with primary breast cancer, cohabiting with a male partner, having no previous cancer diagnoses, receiving no neo-adjuvant treatment, having no history of hospitalisation due to psychosis, and able to read and speak Danish. Partners were eligible if they could read and speak Danish and were ≥ 18 years. DISCUSSION: This study investigates the effect of an attachment-oriented psychological intervention for breast cancer patients and their partners. The intervention has a theoretical framework and a strong design. If proven effective, this intervention would be helpful in optimising psychosocial care and rehabilitation of couples coping with breast cancer. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT01368380.