Objective : (1) To investigate the effect of probiotic bacterium Lactobacillus paracasei SD1 on the level of salivary mutans streptococci and lactobacilli, and (2) the oral persistence of L. paracasei SD1 in orthodontically treated nonsyndromic cleft lip and palate patients. Design : Double-blinded, randomized, placebo-controlled study. Participants : A total of 30 orthodontically treated nonsyndromic cleft lip and palate patients (aged 19.22 ± 3.66 years): 15 in the intervention group (A) and 15 in the control group (B). Interventions : Milk with or without L. paracasei SD1. Average daily consumption of milk in both groups was 50 mL for 4 weeks. Main Outcome Measures : Salivary mutans streptococci and lactobacilli were enumerated using a quantitative differential culture at baseline and once a week after the end of the administration period for 4 weeks. The persistence of L. paracasei SD1 was traced using arbitrarily primed polymerase chain reaction of the DNA fingerprinting. Results : A statistically significant reduction in mutans streptococci counts occurred in group A, in contrast to group B (P < .001). A significant increase of lactobacilli numbers was found in group A (P < .001), and L. paracasei SD1 could be detected up to 4 weeks following cessation of dosing. Conclusions : The probiotic milk powder containing L. paracasei SD1 could reduce mutans streptococci counts and was apparently able to colonize the oral cavity of the orthodontically treated cleft lip and palate patients. However, the potentially beneficial influence of the probiotic milk on the complex oral microflora justifies further studies with a larger group of volunteers.