Background: The most used 1^st line (1L) chemotherapy backbone for mCRC is fluoropyrimidine (FP) + oxaliplatin. In the 2L, iri alone or with 5FU (FOLFIRI), ± a biologic agent (biologic), are options. It is unclear whether adding 5FU to iri is beneficial following disease progression on prior FP. Methods: This study used the nationwide Flatiron Health electronic health record (EHR)-derived de-identified database to identify patients diagnosed with mCRC from 01-01-2013 to 02-13-2023, who received 1L with FP + oxaliplatin ± biologic, and 2L with irinotecan ± 5FU ± biologic. Baseline patient/tumor characteristics were obtained. The number of major comorbidities was computed using ICD-9-CM/10 codes within 1 year prior to 2L (Quan, 2005). Time to next treatment (TTNT) was defined from the start of 2L to the start of next line treatment (excluding maintenance). For survival outcomes, the median survival time and 95% confidence interval (CI) were estimated using the Kaplan-Meier method. Uni- and multi-variable analyses (UVA and MVA) were performed with the cox proportional hazard model. All covariates were evaluated in UVA, and those significant were included in MVA with adjusted hazard ratio (HR) and 95% CI. Results: 3,215 patients met criteria; most previously received a biologic with 1L (70.5% prior anti-angiogenesis and 5.9% prior anti-EGFR) and had 0 structured major comorbidity diagnoses (82.1%). In the 2L, FOLFIRI was the most used chemotherapy backbone (90.6%, vs iri 9.4%), 79% of patients also received a biologic, and patients who received FOLFIRI were younger (median age 61 vs 65 years) and had better ECOG performance status (87.8% vs 81.8% ECOG 0-1). Overall survival (OS) was longer with FOLFIRI than iri (median 14.4 vs 9.8 months, p<0.001), as was TTNT (median 7.6 vs 5.3 months, p<0.001). In MVA, when controlling for age, comorbidities, CEA, ECOG performance status, and 1L, iri was associated with shorter OS than FOLFIRI (HR 1.38, 95% CI 1.21-1.57, p<0.001). In MVA, the addition of a biologic to FOLFIRI and iri prolonged OS and TTNT in some subgroups (Table). Conclusions: In this retrospective real-world analysis of 3,215 patients with mCRC who received 1L with FP + oxaliplatin ± biologic, 2L FOLFIRI was associated with longer OS compared to iri alone (both ± biologic).