OBJECTIVE: To examine whether xylitol, which reduces the growth of Streptococcus pneumoniae, might have clinical importance in the prevention of acute otitis media. DESIGN: A double blind randomised trial with xylitol administered in chewing gum. SETTING: Eleven day care nurseries in the city of Oulu. Most of the children had had problems with recurrent acute otitis media. SUBJECTS: 306 day care children: 149 children in the sucrose group (76 boys; mean (SD) age 4.9 (1.5) years) and 157 in the xylitol group (80 boys; 5.0 (1.4) years). INTERVENTION: Either xylitol (8.4 g a day) or sucrose (control) chewing gum for two months. MAIN OUTCOME MEASURES: The occurrence of acute otitis media and antimicrobial treatment received during the intervention and nasopharyngeal carriage of S pneumoniae. RESULTS: During the two month monitoring period at least one event of acute otitis media was experienced by 31/149 (20.8%) children who received sucrose compared with 19/157 (12.1%) of those receiving chewing gum containing xylitol (difference 8.7%; 95% confidence interval 0.4% to 17.0%; P = 0.04). Significantly fewer antimicrobials were prescribed among those receiving xylitol: 29/157 (18.5%) children had at least one period of treatment versus 43/149 (28.9%) (difference 10.4%; 0.9% to 19.9%; P = 0.032). The carriage rate of S pneumoniae varied from 17.4% to 28.2% with no difference between the groups. Two children in the xylitol group experienced diarrhoea, but no other adverse effects were noted among the xylitol users. CONCLUSION: Xylitol seems to have a preventive effect against acute otitis media.