Background: Safety-net adults generate a high rate of emergency department (ED) visits within the 180 days after a new cancer diagnosis, many of which could be alternatively triaged to an urgent care clinic. It is unclear how much of this ED use is attributable to the cancer and treatment vs. ED-seeking behavior. To identify patients at risk of frequent ED use, we explored whether a patient’s pre-cancer ED visit use predicted ED use after diagnosis. Methods: We identifiably linked adults from the tumor registry in the Dallas County safety-net health system to a regional hospital database with claims-like data for all patients from 98% of non-federal hospitals in North Texas. We applied a mixed-effects multivariate logit model, using frequent ED use (≥4 visits) in the 6-12 months or 12-18 months before diagnosis to predict frequent ED use after diagnosis, adjusting for demographics, comorbidities; cancer type, stage, initial treatment modalities; and grouping visits at the patient level. Results: Of 8,610 adults diagnosed from 2012-2016, 76.2% had Medicaid or were uninsured, 30.9% had lung, breast, or colorectal cancer, and 25.9% had advanced-stage cancer at diagnosis. In the 180 days after diagnosis, 42.5% of patients had zero ED visits, 45.7% had 1-3 visits, and 11.8% were frequent ED users (≥4). In multivariate analysis, patients with frequent ED use in the 6-12 months before a cancer diagnosis had 6.7 higher odds (95% CI: 4.8, 9.3) of having frequent ED use after diagnosis, compared to patients who had zero ED visits prior to diagnosis. This compared to 1.3 higher odds (95% CI: 1.1, 1.5) of frequent ED use if the patient had advanced-stage cancer, and 2.1 higher odds (95% CI: 1.8, 2.4) if chemotherapy was part of initial treatment. Although most post-diagnosis frequent ED users generated zero visits (62.2%) or 1-3 visits (30.7%) in the 6-12 months prior to diagnosis, 38% of patients with frequent ED use pre-diagnosis continued frequent ED visits after diagnosis. Results were similar for ED use 12-18 months prior to diagnosis. Conclusions: Among safety-net adults, prior ED-seeking behavior strongly predicted ED use after a new cancer diagnosis. This may represent a high-risk group that might benefit from care delivery innovation.