Background <p>Antibiotics are frequently prescribed for acute upper respiratory infections (URIs) despite limited evidence of their effectiveness, particularly in older individuals. Understanding the potential associations of such prescriptions with subsequent health outcomes is critical for clinical decision-making. This study investigated the association between antibiotic prescription and pneumonia diagnosis among older adults with acute URIs in the Republic of Korea.</p> Methods <p>We conducted a retrospective analysis using national health insurance claims data from January 1, 2018, to December 31, 2018. The study included 5,898,239 prescription units from 3,059,864 patients aged ≥ 65 years diagnosed with acute URIs. Antibiotic prescription status, pneumonia diagnoses, and patient characteristics such as age, sex, and comorbidities were analyzed. Multivariate logistic regression was used to evaluate associations with pneumonia diagnoses.</p> Results <p>Of all prescription units analyzed, 34.4% included antibiotics. Pneumonia was diagnosed in 7.1% of all units, with a higher proportion in the antibiotic group (8.1%) than in the non-antibiotic group (6.6%). After adjusting for age, sex, and comorbidities, antibiotic prescription was associated with a 1.3-fold higher likelihood of pneumonia diagnosis. Higher odds of pneumonia were also observed in men, older individuals, and those with chronic conditions such as chronic obstructive pulmonary disease, heart failure, cancer, ischemic heart disease, and diabetes.</p> Conclusions <p>Among older adults with acute URIs, antibiotic prescriptions were associated with a higher prevalence of pneumonia diagnoses. These findings highlight the importance of cautious antibiotic prescribing in this population and consideration of individual comorbidities.</p>