In this case study, the process of nutritional diagnosis and intervention conducted at a hospital on a malnourished patient who underwent treatment for a chronic illness (chemotherapy for cancer treatment) was recorded. The patient received his first round of chemotherapy for colorectal cancer, and then a second round after the cancer metastasized to the liver. The patient was malnourished and had experienced weight loss (17% loss in the most recent 3 months) due to side effects of chemotherapy including stomatitis, nausea, and vomiting. Nutritional diagnosis and intervention via the nutrition care process were implemented through two screening rounds, and the quantity of oral intake increased from 28% to 62% of the recommended daily intake. The patient required continuous monitoring and outpatient care after hospital discharge. It is speculated that if a more active patient education and dietary regimen with respect to chemotherapy side effects had been offered after the patient's first chemotherapy cycle, it might have been possible to treat ingestion problems due to stomatitis during the second cycle of chemotherapy and prevent the weight loss. Henceforth, patients receiving chemotherapy should be educated about nutrition management methods and monitored continuously to prevent malnutrition.