The purpose of this study was to evaluate the risk of caries in orthodontic patients before and after active treatment. One hundred and three patients (17 males, 86 females), who were treated in the Obihiro Art orthodontic clinic between 2000 and 2007, participated in the study. The mean age at the first examination was 19.4 years old (range : 8-51 years old). A caries risk test was carried out on all subjects at the first examination and at the end of active treatment. The test consisted of eight items, including volume of saliva secreted over 5 minutes, salivary buffering capacity, number of colonies of Streptococcus mutans, number of colonies of Lactobacillus, eating frequency per day, amount of plaque attached to teeth, daily usage of fluoride toothpaste and/or fluoride gargles and DMFT. From the results of a Wilcoxon signed-rank test, four items (number of colonies of Streptococcus mutans, eating frequency per day, amount of plaque attached to teeth, and daily usage of fluoride toothpaste and/or gargles) at the end of active treatment significantly lowered the risk of caries (P<.01). The other four items showed no difference in caries risk before or after active treatment. From the results of Spearman's rank correlation, three pairs (volume of saliva and salivary buffering capacity, number of Streptococcus mutans and of Lactobacillus, number of Streptococcus mutans and amount of attached plaque) were significantly correlated at both periods. The results of this study strongly suggest that orthodontic treatment simultaneously reduces the caries risk of the patient. This is thought to be due to the morphological and functional improvement of malocclusion, as well as encouragement of the patient's motivation for oral hygiene.