ObjectiveTo investigate the regular use of xylitol, compared with sorbitol, to prevent acute otitis media (AOM), upper respiratory tract infections (URTIs) and dental caries.DesignBlinded randomised controlled trial with a 6-month study period.SettingEnrolment took place at 11 primary care practices in Ontario, Canada.PatientsChildren aged 1–5 years who did not use xylitol or sorbitol at enrolment.InterventionsChildren were randomly assigned to use a placebo syrup with sorbitol or xylitol syrup two times per day for 6 months.Main outcome measuresPrimary outcome was the number of clinician-diagnosed AOM episodes over 6 months. Secondary outcomes were caregiver-reported URTIs and dental caries.ResultsAmong the 250 randomised children, the mean (SD) age was 38±14 months and there were 124 girls (50%). There were three clinician-diagnosed AOM episodes in the 125 placebo group participants and six in the 125 xylitol group participants (OR 2.04; 95% CI 0.43, 12.92; p=0.50). There was no difference in number of caregiver-reported URTI episodes (rate ratio (RR) 0.88; 95% CI 0.70, 1.11) between the placebo (4.2 per participant over 6 months; 95% CI 3.6, 5.0) and xylitol (3.7; 95% CI 3.2, 4.4) groups. Dental caries were reported for four participants in the placebo group and two in the xylitol group (OR 0.42; 95% CI 0.04, 3.05; p=0.42). In a post-hoc analysis of URTIs during the COVID-19 pandemic, the rate among the 59 participants receiving placebo was 2.3 per participant over 6 months (95% CI 1.8, 3.0) and for the 55 receiving xylitol, 1.3 over 6 months (95% CI 0.92, 1.82; RR 0.56; 95% CI 0.36, 0.87). The most common adverse event was diarrhoea (28% with placebo; 34% with xylitol).ConclusionsRegular use of xylitol did not prevent AOM, URTIs or dental caries in a trial with limited statistical power. A post-hoc analysis indicated that URTIs were less common with xylitol exposure during the COVID-19 pandemic, but this finding could be spurious.Trial registration numberNCT03055091.