A novel slow-release administration device, the "Fall-Asleep Pacifier" (FAP), was studied as a prophylactic measure against mutans streptococcal oral infection and dental caries in a risk group of 1-year-old children by comparing the test (T, n = 34) and control (C, n = 88) groups in a prospective cohort study. In the T group the children received their fluoride tablets (Fludent, containing NaF corresp. 0.25 mg F0- , xylitol 159 mg and sorbitol 153 mg) in the evenings in FAP. In the C group the children received the same dose of Fludent crushed in food in the evenings. The proportion of children, whose plaque samples from the upper incisors were mutans streptococcus positive at the age of 24 months, was significantly (P < 0.05) greater in group C (25%) than in group T (9%). The children in the T group developed significantly (P < 0.001) less (none) new dentinal carious lesions in their primary dentitions than the children in the C group between 2 and 3 1/2 years of age. Fifty-four percent of the children to whom the FAP was offered complied with regular use of it. The beneficial effect observed in the T group compared with the C group was apparently mostly due to the administration mode via FAP, which could prolong the intra-oral bioavailability of the prophylactic preparation.