Background: While overall incidence and mortality of colorectal cancer (CRC) has declined, incidence and mortality are increasing in those < 50 years old (early-onset CRC). Our objective was to better understand early-onset CRC by comparing tumor characteristics and initial treatment type between early-onset and normal/late-onset CRC. Methods: We used health system and national tumor registries to identify patients diagnosed with adenocarcinoma of the colon or rectum from 2010-2014 at 6 US integrated health systems in the Patient Outcomes To Advance Learning (PORTAL) network. Tumor registry data included: age at diagnosis, stage, grade, anatomic site, histology, number of lymph nodes examined, and receipt of initial systemic therapy (chemotherapy or immunotherapy). Demographics and other patient characteristics were obtained from the EHR. We used logistic regression to calculate adjusted odds ratios (ORs) and 95% confidence intervals (CIs) comparing the distribution of tumor characteristics and treatment patterns in early-onset ( < 50 years old) vs. normal/late-onset CRC. Results: There were 1,424 early-onset and 10,810 normal/late onset CRC cases in our analyses. Compared to normal/late onset cases, patients with early-onset CRC were more likely to be Hispanic, obese, never smokers, and to have Charlson comorbidity scores < 3. After adjustment for patient characteristics, compared to normal/late onset CRC, early-onset CRC was associated with more advanced stage disease (OR for stage 4 vs. stage 1 = 2.8, CI: 2.4-3.4), high grade histology (OR for poorly differentiated/undifferentiated vs. well/moderately differentiated = 1.2, CI: 1.1-1.5), signet ring histology (OR for signet ring vs. non-mucinous adenocarcinoma = 1.7, CI: 1.1, 2.6), and rectal (OR for rectum vs. cecum = 2.4, CI: 1.9-2.9) or left colon location (OR for left colon vs. cecum = 2.2, CI: 1.8-2.8). After adjustment for patient and tumor characteristics, early-onset patients were more likely than normal/late onset patients to have > 12 lymph nodes examined (OR = 1.6, CI: 1.4-1.8) and to receive systemic therapy (OR = 2.8, CI: 2.4, 3.4). Conclusions: Early-onset CRC is associated with aggressive tumor characteristics, distal location, and systemic therapy use.