Urinary tract infection (UTI) is a common health care-associated adverse event and the leading nosocomial complication following pediatric urological surgery. While continuous antimicrobial prophylaxis effectively reduces the risk of UTI following such a surgery, non-adherence is common and represents a distinct clinical entity that is associated with renal scarring. Acceptability is likely to have a significant impact on patient adherence. Herein we used a validated data-driven approach—the ClinSearch acceptability score test (CAST)—to investigate the acceptability of cefaclor, an oral antibiotic widely used for the prevention of pediatric UTI in Japan. Standardized observer reports were collected for 58 intakes of cefaclor 10% fine granules in patients aged from 0 to 17 years. The medicine was classified as positively accepted on the acceptability reference framework. According to the percentage of the prescribed dose taken reported at the end of the treatment, patients exhibited good adherence to this well-accepted medicine. Nonetheless, requirements for greater dosing frequency or poor acceptability in certain patients could affect adherence. Acceptability should be established to ensure patient adherence to medicines used for long-term prophylaxis and consequently guarantee the safety and efficacy of the treatment.