Background: The use of telemedicine among cancer patients remains limited. Because of the COVID-19 pandemic, CMS expansion of telehealth with 1135 waiver was enacted in March 2020, broadening the opportunities to provide patient care using telemedicine. This study examined the impact of the CMS expansion on the use of telemedicine among cancer patients. Methods: We identified newly diagnosed patients with 5 common cancers (breast, prostate, lung, colorectal, and lymphoma) between 3/2019 and 12/2020 from Optum’s de-identified Clinformatics Data Mart Database. Patients who had 6 months of full enrollment (3 months before and 3 months after the first [index] cancer diagnosis date), had cancer claims on 3 separate dates within 3 months of the index date for the specific cancer diagnosis, and no prior history of cancer were included. We defined telemedicine use as patients who had a telemedicine procedure code within 1 month of their index diagnosis and had the cancer diagnosis on the telemedicine claim. We conducted an interrupted time series analysis to examine the impact of CMS expansion on telemedicine use. A multivariable logistic regression model was used to identify factors associated with telemedicine use during the post-expansion period. Results: Of 96,632 patients included, the average crude rate of telemedicine use was 0.12% before and 14.2% after the expansion in March 2020 (see Table). There was a significant impact of expansion on telemedicine use (21% increase; p<0.001). The peak rate (adjusted) was 28% in April 2020, decreasing and plateauing in July/August 2020, with rates staying in the range of 10-12% between August and December 2020. During the post-expansion period, lymphoma, prostate, and lung cancer patients (adjusted rates: 14.6%, 15.7%, and 15.9%, respectively) were more likely to use telemedicine compared to patients who had breast (12.7%) or colorectal (12.3%) cancer. Patients who were older (adjusted rates: ≥65 years, 13.8%; 50-64, 14.2%; 20-49, 18.6%), Black (12.4% vs 14.4% for White, 15.5% for Hispanic and 16.6% for Asian), resided in East South Central census division (8.4% vs 23.5% in New England) and had Medicare (12.2% vs 20.3% for commercial insurance) were less likely to use telemedicine (all p<.001). Conclusions: After the CMS telehealth expansion, the use of telemedicine among newly diagnosed cancer patients increased significantly. Telemedicine use varied by patient age, geographic location, race/ethnicity, and payer. Further research is needed to understand the pattern of telemedicine use.