We evaluated the clinical evidence regarding probiotic use for the prevention of respiratory tract infections (RTIs). Randomised controlled trials (RCTs) studying the effects of probiotics for the prevention of upper or lower RTIs were systematically identified. Fourteen RCTs (twelve involving healthy subjects and two involving patients with RTIs) were included. Various Lactobacillus strains were used in seven RCTs, combinations of Lactobacillus and Bifidobacterium strains were used in five RCTs, and a Bifidobacterium strain and a non-pathogenic Enterococcus faecalis strain were used in one RCT, respectively. In ten RCTs no difference was found regarding the incidence of RTIs in the probiotic arm compared with the control arm, whereas the remaining four RCTs favoured the use of probiotics. Reduction in the severity of symptoms related to RTIs was noted in five of six RCTs that provided relevant data. In three of nine RCTs that provided relevant data, the clinical course of RTIs was shorter in the probiotic arm, whereas no difference was found in the remaining six RCTs. In conclusion, probiotics may have a beneficial effect on the severity and duration of symptoms of RTIs but do not appear to reduce the incidence of RTIs.