86 Background: Weight loss is prevalent among cancer patients and correlates with lower health-related quality of life and overall survival rates. Individuals diagnosed with head and neck (HN) and pancreatic cancers face a heightened risk of weight loss due to tumor location and treatment-related side effects. Although guidelines advocate early interventions such as referral to a dietitian for tailored nutritional support, the precise impact of oncology nutrition services (ONS) on weight loss remains unclear. In this study, we investigated the longitudinal association between weight change and ONS among patients diagnosed with HN or pancreatic cancers. Methods: A retrospective chart review was conducted for patients diagnosed with HN or pancreatic cancers visited between January 2019 and December 2023. Inclusion criteria encompassed patients who had weight measurements taken within 30 days of their cancer diagnosis. Patient demographics, clinical characteristics, all recorded body weights, and receipt of ONS were extracted from electronic health records. Weight change (%) from the baseline was calculated at each measurement by subtracting the initial weight value. A mixed-effects model was employed to evaluate the impact of ONS on weight change, with time to weight measurement and ONS serving as interaction terms, after adjusting for demographics and clinical characteristics. Results: A cohort of 1305 patients was identified, predominantly comprising males (n=1014, 63%) and individuals of White race (n=1358, 85%), with a diverse ethnic distribution (Hispanic: n=620, 39%). Of these patients, 947 (73%) received ONS at any point following cancer diagnosis. Significant weight loss over time was noted among all patients (-0.53% per 6 months, p<0.001). Baseline weight, sex, ethnicity, receipt of percutaneous endoscopic gastrostomy, total parenteral nutrition, radiation, chemotherapy, and ONS were significantly associated with weight loss. Following adjustment for the covariates, patients receiving ONS showed significantly less weight loss compared to those without ONS (-0.06% [95% confidence interval (CI): -0.10, -0.03] vs. -0.04% [95% CI: -0.38, -0.23] per 6 months, p<0.001). Conclusions: The findings indicate that ONS plays a significant role in attenuating weight loss among patients diagnosed with HN or pancreatic cancer, irrespective of baseline demographics or clinical status. While the magnitude of weight change may appear modest, its implications are substantial long term. Early referrals to ONS should be recommended for HN or pancreatic cancer patients to mitigate weight loss.