Abstract Background: ASCO Clinical Practice Guidelines on fertility preservation (FP) recommend that healthcare providers discuss the possibility of infertility—including FP options—for cancer patients of reproductive-age as early as possible before treatment initiation. However, the evaluation of patient perceptions on provider adherence to these FP guidelines is largely undetermined. We sought to assess patient-reported provider adherence to FP guidelines for adults ages 18 to 49 years when diagnosed with a first primary cancer (early-onset). Patients and Methods: The Reproductive Health After Cancer Diagnosis and Treatment (REACT) Study is a patient-partnered study in cooperation with 23 community partners and patient advocates. Adults with a first primary early-onset cancer were recruited over an 8-week period (October to December 2021). Patient-reported provider adherence to FP guidelines, reproductive health history, sociodemographic, cancer and cancer impact questions were prospectively collected via patient questionnaire using a secure, web-based application. To examine the association of provider non-adherence with patient-level factors, logistic regression models were fitted to estimate odds ratios (OR) and 95% CI. Results: Among 647 patients diagnosed with early-onset cancer between 2006 and 2021 (mean [SD] age, 36.8 [7.8] years), a total of 376 (58.1%) reported that a healthcare professional involved in their cancer care did not discuss FP options before starting cancer treatment. Patients with thyroid, lung, ovarian and colorectal cancers reported the highest rates of provider non-adherence to FP guidelines (all ≥63% non-adherence). Females with cervical (OR 2.87, 95% CI: 1.11-7.41), colorectal (OR 2.05, 95% CI: 1.15-3.68), lung (OR 15.79, 95% CI:5.66-44.1) and ovarian (OR 10.90, 95% CI: 3.19-37.3) cancers, Hodgkin (OR 4.00, 95% CI: 1.15-13.98) and non-Hodgkin lymphoma (OR 4.65, 95% CI: 1.11-19.47) were more likely to report provider non-adherence to FP guidelines compared to breast cancer patients in models adjusted for diagnosis age and year, self-reported race/ethnicity, marital status and insurance coverage at cancer diagnosis. Similar patterns were observed for male patients with early-onset colorectal and lung cancers compared with testicular cancer patients. No overall or sex-specific associations were observed between patient-reported provider non-adherence and race/ethnicity, insurance coverage or diagnosis year. Conclusions: Nearly six of every 10 early-onset cancer patients reported that a healthcare professional involved in their cancer care did not discuss FP options prior to cancer treatment. The development of tailored, effective strategies for implementation of clinical practice guidelines on FP and delivery of concordant reproductive healthcare to a growing population of reproductive-age individuals with cancer is needed. Citation Format: Samantha R. Keller, Mark A. Lewis, Allison Rosen, Hyo K. Park, Rebecca Babyak, Jill Feldmann, Fei Ye, Rajiv Agarwal, Kristen K. Ciombor, Timothy M. Geiger, Cathy Eng, Michelle K. Roach, Digna R. Velez Edwards, Michele L. Cote, Richard Viskochil, Andreana N. Holowatyj. Limited adherence to clinical practice guidelines on fertility preservation for patients with early-onset cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2233.