OBJECTIVES: To examine patient characteristics, healthcare resource utilization (HRU) and medical costs of patients who were untreated for neuroendocrine tumors (NETs).  METHODS: Using a US administrative claims database, commercially-insured adults newly diagnosed with carcinoid tumors (ICD-9-CM: 209.xx) or pancreatic islet cell tumors (ICD-9-CM: 157.4 and 211.7) between 07/01/2007 and 12/31/2010 were identified (the first observed diagnosis date as the index date). Patients were required to have 6-month pre-index and 12-month post-index enrollment. Untreated patients were defined as those receiving neither surgical nor medical treatments in the 12-month follow-up period. Descriptive analysis was performed to assess evolution of HRU and medical costs over time. Similar analysis was conducted for Medicare-eligible patients.  RESULTS: Of the 3,940 commercially-insured and 1,658 Medicare-eligible individuals with NETs, 63.0% (n=2,484) and 67.0% (n=1,111) were untreated, respectively. Among untreated commercially-insured individuals with NETs, carcinoid syndrome (20.9%), nausea/vomiting (14.2%) and liver metastasis (11.6%) were the most prevalent symptoms/co-morbidities in the 12-month post-index period; 37.7 % had hospitalization admissions and 31.4% had emergency department (ED) visits, and the mean annual number of physician office visits was 18.7. The total monthly medical cost increased from $3,028 in the pre-index period to $4,159 in the post-index period. Among untreated Medicare-eligible individuals with NETs, carcinoid syndrome (14.3%), nausea/vomiting (12.5%) and liver metastasis (11.9%) were the most prevalent symptoms/co-morbidities in the 12-month post-index period; 42.2 % had hospitalization admissions and 35.0% had ED visits, and the mean annual number of physician office visits was 25.5. The total monthly medical cost increased from $2,787 in the pre-index period to $3,788 in the post-index period.  CONCLUSIONS: Economic burden of untreated individuals with NETs is not negligible in the healthcare system. Future research should assess reasons for non-treatment, the impact of non-treatment on quality of life, and the benefit to cover this population with medical unmet need.