AIMS: To assess evidence of health and immune benefit by consumption of a Lactobacillus casei Shirota probiotic in highly physically active people. METHODS: Single-centre, population-based, randomized, double-blind, placebo-controlled trial. Daily ingestion of probiotic (PRO) or placebo (PLA) for 20 weeks for n = 243 (126 PRO, 117 PLA) university athletes and games players. Subjects completed validated questionnaires on upper respiratory tract infection symptoms (URS) on a daily basis and on physical activity status at weekly intervals during the intervention period. Blood samples were collected before and after 20 weeks of the intervention for determination of Epstein Barr virus (EBV) and cytomegalovirus (CMV) serostatus and antibody levels. RESULTS: URS episode incidence was unexpectedly low (mean 0.6 per individual) and was not significantly different on PRO compared with PLA. URS episode duration and severity were also not influenced by PRO. A significant time x group interaction effect was observed for plasma CMV antibody titres in CMV seropositive participants (p < 0.01) with antibody titre falling in the PRO group but remaining unchanged in the PLA group over time. A similar effect was found for plasma EBV antibody titres in EBV seropositive participants (p < 0.01) with antibody titre falling in the PRO group but increasing in the PLA group over time. CONCLUSIONS: In summary, regular ingestion of PRO did not reduce URS episode incidence which might be attributable to the low URS incidence in this study. Regular ingestion of PRO reduced plasma CMV and EBV antibody titres, an effect that can be interpreted as a benefit to overall immune status.