1092 Background: Response Evaluation Criteria in Solid Tumors (RECIST) criteria are often used to measure tumor response in cancer trials, especially in the stage IV setting. However, RECIST requires measurable disease, which is less common in invasive lobular breast carcinoma (ILC) of the breast, a diffusely growing tumor type, compared to invasive ductal carcinoma (IDC). We examined the prevalence of RECIST in breast cancer clinical trials, and whether there are differential trial enrollment rates by histology and stage. Methods: We analyzed the clinicaltrials.gov database to evaluate the proportion of interventional, stage IV clinical trials that require measurable disease as inclusion criteria or outcome measures. We then performed an institutional cohort study comparing the proportion of patients in the University of California, San Francisco (UCSF) OnCore clinical trials management system (CTMS) to the UCSF Cancer Registry between 2000-2018, stratified by histology and stage. We hypothesized that the proportion of patients with ILC in the CTMS would be significantly lower than in the cancer registry. Results: There were 146 actively-recruiting, interventional clinical trials for stage IV breast cancer that were identified in our search on clinicaltrials.gov. Overall, 108 (74%) required measurable disease for study participation. The UCSF Cancer Registry included 8,679 patients, while the UCSF OnCore CTMS included 1,511 patients (Table). In those with early stage disease, where RECIST is not typically used, there was no difference in the proportion of ILC patients enrolled in clinical trials versus in the cancer registry. However, among those with stage IV disease, there was a significantly lower proportion of patients with ILC in the CTMS than in the cancer registry (9.2% versus 17.9%, p = 0.005). In contrast, patients with stage IV IDC were overrepresented in the clinical trials database compared to the cancer registry. Conclusions: Patients with metastatic ILC were significantly less likely to be enrolled in clinical trials than those with metastatic IDC. This decreased enrollment may be due to the widespread use of RECIST, and further investigation is needed to ensure equity in access to clinical trials.[Table: see text]