Objectives <p>This study evaluated the routine use of upper gastrointestinal (UGI) endoscopy combined with oral-pharynx-larynx surveillance to assess oral hygiene and its impact on pneumonia development in vulnerable patients.</p> Materials and methods <p>This retrospective observational study included 411 hospitalized participants who underwent UGI endoscopy with concurrent oral–pharynx–larynx examination. Poor oral hygiene was identified based on endoscopic findings, including nasopharyngeal pooling secretions, diffuse inspissated oral secretions, and food particles, tartar, or plaque. Among these, 68 participants were identified with poor oral hygiene, while the remaining 343 participants comprised the control group. Records of basic and clinical characteristics were analyzed.</p> Results <p>Gastroenterologists identified 2.6% (68/2,567) of participants with poor oral hygiene during endoscopy. The frequency of pneumonia was notably higher in the poor oral hygiene group (3.0 ± 1.5 episodes/person-year) compared to the control group (0.6 ± 2.0 episodes/person-year, <i>p</i> &lt; 0.001). Multivariable analysis showed that poor oral hygiene significantly increased the risk of pneumonia requiring hospitalization (<i>p</i> &lt; 0.001). Enteral tube feeding was also independently associated with increased pneumonia risk (<i>p</i> = 0.003). Kaplan–Meier analysis indicated a significantly higher cumulative incidence of pneumonia in the poor oral hygiene group.</p> Conclusions <p>Poor oral hygiene identified during the endoscopic examination was significantly associated with an increased risk of pneumonia requiring hospitalization.</p> Clinical relevance <p>Gastroenterologists can identify vulnerable patients with poor oral hygiene during endoscopy, providing an opportunity for referral to dentists or primary caregivers for timely intervention and management.</p>