Introduction: Caring for a significant other during cancer treatment can be demanding. Little is known about the well-being of informal caregivers of patients with colon cancer. This study aims to examine informal caregiver well-being during adjuvant chemotherapy for colon cancer. Material and methods: This exploratory longitudinal, prospective study measured the course of informal caregiver burden (Self-Perceived Pressure of Informal Care), distress (Hospital Anxiety and Depression Scale), health-related quality of life (RAND-36), marital satisfaction (Maudsley Marital Questionnaire), social support (Social Support List – Discrepancies), fatigue (Abbreviated Fatigue Questionnaire), and self-esteem (Caregiver Reaction Assessment) before (T0), during (T1), and after (T2) patients’ treatment. Results: Baseline data of 60 out of 76 eligible dyads (79%) were analyzed. Mean levels of informal caregiver burden and distress improved significantly over time, as did their health-related quality of life and perceived social support. At baseline, 30% and 26.7% of informal caregivers reported moderate-to-high levels of burden and clinically relevant levels of distress, respectively, which changed to 20% and 18.8% at T2. Informal caregiver burden and distress at baseline were the strongest predictors of informal caregiver burden and distress during and following patients’ treatment, respectively. Conclusion: When informal caregivers and patients experience problems before start of adjuvant chemotherapy, problems seem to improve over time. Approximately 20% of informal caregivers remain burdened and distressed after patients’ end of treatment. Paying attention to baseline distress and burden seems indicated, as these were strong predictors of informal caregivers’ well-being during and after treatment.