Background: Support from the family positively affects self-management, patient outcomes and the incidence of hospitalizations among patients with heart failure. To involve family members in heart failure care is thus valuable for the patients. Registered nurses frequently meet family members of patients with heart failure and the quality of these encounters is likely to be influenced by the attitudes registered nurses hold toward families.Aims: To explore registered nurses' attitudes toward the importance of families' involvement in heart failure nursing care and to identify factors that predict the most supportive attitudes.Methods: Cross-sectional, multicentre web-survey study. A sample of 303 registered nurses from 47 hospitals and 30 primary health care centres completed the instrument Families' Importance in Nursing Care - Nurses' Attitudes.Results: Overall, registered nurses were supportive of families' involvement. Nonetheless, attitudes toward inviting families to actively take part in heart failure nursing care and involve families in planning of care were less supportive. Factors predicting the most supportive attitudes were to work in a primary health care centre, a heart failure clinic, a workplace with a general approach toward families, to have a postgraduate specialization, education in cardiac and/or heart failure nursing care, and a competence to work with families.Conclusions: Experienced registered nurses in heart failure nursing care can be encouraged to mentor their younger and less experienced colleagues to strengthen their supportive attitudes toward families. Registered nurses who have designated consultation time with patients and families, as in a nurse-led heart failure clinic, may have the most favourable condition for implementing a more supportive approach to families.