Background: Mixed Invasive Ductal-Lobular Carcinoma (IDC-L) is a histological subtype of invasive breast carcinoma comprised of both ductal and lobular morphologies. There is limited information on the relative proportions of the individual components in IDC-L and on outcomes compared to invasive lobular carcinoma (ILC) and invasive ductal carcinoma (IDC). Methods: Clinical information was abstracted from 16,308 patients with invasive breast cancer seen at UPMC Magee Women’s Hospital from 1990-2017 using the UPMC Network Cancer Registry. A systematic chart review was performed on a subset of patients annotated with IDC-L (n=806); however, a thorough review of pathology reports led to the exclusion of all but 408 patients for further analysis, due to the lack of a standardized definition of IDC-L. Of the 408 cases, 92% were estrogen receptor (ER)+. Survival of patients with ER+ IDC-L (n=376) was compared to ER+ IDC (n=9,716) and ER+ ILC (1,465). For a subset of IDC-L cases (n=54), distributions of individual subtype components were abstracted from pathology reports. Results: IDC-L made up 2.5% of the total cases (408/16,308). IDC-L tumors were on average 31% ductal and 69% lobular (p =0.001). Survival analysis showed worse disease free survival (DFS) (p=0.05) and overall survival (OS) (p=0.002) in patients with ER+ ILC compared to ER+ IDC, with ER+ IDC-L patients showing a median OS superior to ILC yet inferior to IDC counterparts (ns). Conclusions: Identification of patients with IDC-L through cancer registry protocols representative of standard practices by national cancer registries revealed a lack of a standardized definition of mixed IDC-L. Reliance on accuracy of these diagnoses calls in to question the reliability of prior clinico-pathologic analyses reported on this topic. DFS and OS of IDC-L patients falls between that of IDC and ILC patients while ILC patients showed significantly worse outcome. The predominant distribution of lobular morphology in IDC-L tumors suggests this subtype may have additional characteristics similar to lobular rather than ductal carcinomas. Comprehensive clinical and molecular characterization of a carefully identified IDC-L cohort is underway.