This study examined the effect of a probiotics supplementation on respiratory tract infection (RTI) and immune and hormonal changes during the French Commando training (3-week training followed by a 5-day combat course). Cadets (21 +/- 0.4 years) received either a probiotics (n = 24) or a placebo (n = 23) supplementation over the training period. We found no difference in the RTI incidence between groups but a significantly greater proportion of rhinopharyngitis in the probiotic group (p < 0.05). Among immune parameters, the major finding was an immunoglobulin A decrease after the combat course only in the placebo group (p < 0.01), but the difference between the two groups was not significant. A greater increase in dehydroepiandrostane sulfate was observed in the probiotics group after the combat course (p < 0.05). This study suggested that the benefits of a probiotics supplementation in a multistressor environment relied mainly on its capacity to prevent the infection to spread throughout the respiratory tract.