Common infectious diseases (CID) of the airways and the gastrointestinal tract are still a considerable cause of morbidity and mortality in elderly. The present study examined the beneficial effect of a dairy product containing the probiotic strain Lactobacillus casei DN-114 001 (fermented product) on the resistance of free-living elderly to CID. The study was multicentric, double blind and controlled, involving 1072 volunteers (median age = 76·0 years) randomised for consumption of either 200 g/d of fermented (n 537) or control (non-fermented) dairy product (n 535) for 3 months, followed by an additional 1 month's follow-up. The results showed that, when considering all CID, the fermented product significantly reduced the average duration per episode of CID (6·5 v. 8 d in control group; P = 0·008) and the cumulative duration of CID (7 v. 8 d in control group; P = 0·009). Reduction in both episode and cumulative durations was also significant for all upper respiratory tract infections (URTI; P < 0·001) and for rhinopharyngitis (P < 0·001). This was accompanied with an increase of L. casei species in stools throughout the fermented product consumption (2–3·8 × 107 equivalents of colony-forming unit/g of stools, P < 0·001). The cumulative number of CID (primary outcome) was not different between groups nor was the CID severity, fever, pathogens' occurrence, medication, immune blood parameters and quality of life. The fermented product was safe and well tolerated. In conclusion, consumption of a fermented dairy product containing the probiotic strain L. casei DN-114 001 in elderly was associated with a decreased duration of CID in comparison with the control group, especially for URTI such as rhinopharyngitis.