Previous studies have suggested that probiotic supplements in dairy products may affect the oral microbial ecology, but the effect in orthodontic patients has not previously been reported. The aim of the present study was to examine whether short-term consumption of fruit yogurt containing probiotic bifidobacteria would affect the levels of salivary mutans streptococci and lactobacilli in patients with fixed orthodontic appliances. A double-blind, randomized crossover study was performed and 24 healthy adolescents (12-16 years) undergoing orthodontic treatment were followed over four periods. During periods 2 and 4 (2 weeks each), the subjects ingested 200 g fruit yogurt containing Bifidobacterium animalis subsp. lactis DN-173010 (2 x 10(8) colony forming units/g) once daily or a control yogurt without viable bacteria. Periods 1 and 3 were run-in and wash-out periods of 1 and 6 weeks, respectively. Salivary mutans streptococci and lactobacilli were enumerated with chair-side kits before and after the yogurt consumption periods. Pre- and post-treatment values within each regimen were compared with a two-tailed marginal homogeneity test for categorical data. A statistically significant reduction of salivary mutans streptococci was recorded after probiotic yogurt consumption (P < 0.05), which was in contrast to the control yogurt. No significant alterations of the salivary lactobacilli counts were observed. Short-term daily consumption of fruit yogurt containing Bifidobacterium animalis subsp. lactis DN-173010 may reduce the levels of mutans streptococci in saliva during orthodontic treatment with fixed appliances.