<jats:title>Abstract</jats:title><jats:sec><jats:title>Aims</jats:title><jats:p>To explore family members’ experiences when their loved one is undergoing chemotherapy treatment as an outpatient for newly diagnosed colorectal cancer and to develop an explanatory theory of how they process their main concern.</jats:p></jats:sec><jats:sec><jats:title>Background</jats:title><jats:p>Most individuals with cancer are now treated as outpatients and cared for by family members. International research highlights the many side effects of chemotherapy, which in the absence of specific information and/or experience can be difficult for family members to deal with. Unmet needs can have an impact on the health of both patients and family members.</jats:p></jats:sec><jats:sec><jats:title>Design</jats:title><jats:p>Classic grounded theory methodology was used for this study.</jats:p></jats:sec><jats:sec><jats:title>Method</jats:title><jats:p>Using classic grounded theory methodology, family members (<jats:italic>n </jats:italic>=<jats:italic> </jats:italic>35) of patients undergoing chemotherapy treatment for cancer were interviewed (June 2010–July 2011). Data were analysed using the concurrent processes of constant comparative analysis, data collection, theoretical sampling and memo writing.</jats:p></jats:sec><jats:sec><jats:title>Findings</jats:title><jats:p>The main concern that emerged for participants was fear of emotional collapse. This fear was dealt with through a process conceptualized as ‘Emotional Resistance Building’. This is a basic social process with three phases: ‘Figuring out’, ‘Getting on with it’ and ‘Uncertainty adjustment’. The phases are not linear, but interrelated as participants can be in any one or more of the phases at any one time.</jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p>This theory has the potential to be used by healthcare professionals working in oncology to support family members of patients undergoing chemotherapy. New ways of supporting family members through this most difficult and challenging period are articulated within this theory.</jats:p></jats:sec>