AIMS: This study was designed to describe health care resource utilization (HCRU) of patients with metastatic colorectal cancer (CRC) or gastric cancer to test the hypothesis that greater treatment variability would be associated with increased HCRU. METHODS: A retrospective observational study using Marketscan claims data was conducted. Eligible patients had a first diagnosis of metastatic CRC or gastric cancer between 2004 and 2015 and must have received systemic anti-cancer therapy after diagnosis. Treatment variability was measured using the Herfindahl-Hirschman Index (HHI). HHI scores were stratified by quartile. HCRU variables were evaluated throughout the follow-up period and described by 6-month periods. Chi-square test was used for categorical variables and ANOVA for continuous variables. RESULTS: A total of 55,403 CRC and 9,073 gastric cancer patients were eligible. First-line HHI scores ranged from 0.1304-0.2778 for CRC and 0.0383-0.1778 for gastric cancer by state of residence. Statistically significant differences by HHI quartiles for HCRU in CRC included hospitalizations (p = 0.0003), ER visits (p < 0.0001), ER visits leading to hospitalization (p < 0.0001), and supportive care (all agents studied, p < 0.01). For gastric cancer, significant differences by HHI quartile were observed for ER visits (p = 0.002) and selected supportive care (G-CSF, erythropoiesis-stimulating agents, bisphosphonates, nutritional support, and antiemetics, each p < 0.05). No consistent increasing or decreasing trends were observed across the quartiles for either cohort. LIMITATIONS: Large sample sizes could lead to statistical significance without being clinically meaningful. High treatment heterogeneity in the gastric cancer cohort and lack of a homogeneous quartile for comparisons limited the ability to evaluate HCRU by different levels of treatment variability. CONCLUSIONS: Statistically significant relationships were observed between treatment variability as measured by HHI and increased HCRU, but no consistent directional trends in HCRU variables were observed. Therefore, this study failed to reject the null hypothesis of equivalent HCRU by level of treatment variability.