OBJECTIVES: We want to study the real cost structure of cancer treatment using the National Health Insurance Research Database (NHIRD) from Taiwan. METHODS: This study adopted a retrospective observational design and the data were retrieved from the NHIRD, which is managed by the National Health Research Institutes (NHRI). We used descriptive statistical methods to display data and time trend. RESULTS: The number of patients with these five cancers showed an increasing trend in these seven years. Breast cancer had the highest annual increase (7.34%), followed by colorectal cancer (3.75%). Lung cancer and breast cancer had the highest average outpatient cost for each patient per year, while lung cancer, colorectal cancer, and gastric cancer had the highest average inpatient cost for each patient per year. The annual increase rates in the total cancer treatment cost for liver cancer, lung cancer, colorectal cancer, gastric cancer, and breast cancer were 10.70%, 11.61%, 16.73%, 18.38%, and 17.21% respectively.  We divided the cost structure of cancer treatments into chemotherapy, hormone therapy and supportive treatment. The highest percentages of chemotherapy and hormone therapy were for colorectal cancer (more than 95%) and for breast cancer (more than 11%). On average, the annual increase rates in the cancer chemotherapy cost per patient for liver cancer, lung cancer, colorectal cancer, gastric cancer, and breast cancer were respectively 3.59%, 3.66%, 9.86%, 16.78%, and 11.72%. CONCLUSIONS: This study showed a positive correlation between annual cancer patient number and NHI reimbursement, and an increasing trend for chemotherapy, hormone therapy and supportive treatment. The results also showed that the number of the total amount of NHI reimbursement and the drug costs had increasing trends in these seven years. This increased the financial burden of cancer patients and may alter the allocation of NHI resources.