Purpose: The study was done to identify the relationship of treatment side effects, family support, and quality of life in patients with cancer, and factors influencing quality of life. Method: A convenience sample of 106 patients who were receiving cancer treatment at W hospital were interviewed using the Side Effects scale by Hur, Family Support scale by Kang, and QOL scale by Ro. Result: Results indicate that women experienced more severe side effects than men. There was a negative relationship between side effects and quality of life, and a positive relationship between family support and quality of life. The most bothersome side effects were changes in taste and appetite, followed by general weakness and fatigue. Side effects such as loss of hair, nausea, dizziness, numbness, pins and needles in fingers and toes, and dry mouth were also experienced. General weakness and family support were analysed as to whether they were factors influencing quality of life. Conclusion: The results revealed that relieving general weakness should be given high priority in nursing interventions for patients undergoing cancer treatment. In addition, nursing programs should be developed that can reinforce family support.