Background: The role of primary prophylaxis (PP) with granulocyte colony-stimulating factor (G-CSF) for patients with locally advanced gastric cancer (LAGC) treated with FLOT is currently unknown. Although original FLOT4 study did not recommend PP, the use of G-CSF is common in clinical practice to avoid serious complications. However, its use is associated with increased financial costs. Methods: This is a retrospective, single-institution study. We included patients with pathologically confirmed LAGC who received perioperative FLOT4. Patients were categorized in two groups: PP versus no- PP. Our objectives were to compare frequency of grades 3 or 4 neutropenia (G3/4N) and febrile neutropenia (FN) between PP and non-PP patients. We also evaluate the association between pathological complete response (pCR) and disease-free survival (DFS) and overall survival (OS) according to the use of PP. Binomial variables were compared using Fisher's exact test. Kaplan-Meier method was used to calculate median DFS and OS and respective 95% confidence intervals [95%CI] and any difference assessed by the log-rank test. Results: From January 2019 until December 2022, 122 LAGC patients were treated with perioperative FLOT. Median age was 56 years, 53.2% were female and 84.4% were ECOG 0. Primary prophylaxis (PP) was used by 58 patients (47.5%); there was no difference between clinical and tumor features between both groups regarding age, sex, ECOG, T and N staging and Lauren’s subtype. PP costs raised treatment costs up to 15%. Grade 3 or 4 neutropenia occurred more frequently among patients without PP (65.6 vs. 1.7%; p < 0.001). There was no difference in the frequency of FN between groups (7.8 vs. 0%; p = 0.06). Overall patients, pCR occurred in 11.8% os patients. Regarding the influence of PP in pathological response after FLOT chemotherapy, there was no difference between PP vs non-PP patients with pCR (14.3% vs 9.5%, p = 0.57). Median follow-up was 27.4 (range 2.8 – 70.6 months). Twenty-six patients (21.3%) died during follow-up. Median OS was 52.5 months (95% CI: 47.1-57.8). Median OS for patients in PP and no PP groups were 44.5 and 56.2 months (log-rank P =0.06), respectively. Median DFS was 43.1 months (95% CI: 37.2-48.9). Median DFS for patients in PP and no PP groups were 39.6 and 43.5 months (log-rank P =0.80), respectively. Conclusions: Despite the reduction in the frequency of G3/4N, rates of FN among patients treated with FLOT with G-CSF are too low to justify its use in a routine basis. Furthermore, PP is not associated with relevant clinical endpoints such as pCR neither OS or DFS. Based on this, PP in such situations should not be routinely used due to its financial costs.