Probiotics may improve a person's health by regulating their immune function. Some trials have shown that probiotic strains can prevent respiratory infections. Even though the previous version of our review showed benefits of probiotics for acute upper respiratory tract infections (URTIs), several new studies have been published.To assess the effectiveness and safety of probiotics (any specified strain or dose), compared with placebo, in the prevention of acute URTIs in people of all ages, who are at risk of acute URTIs.We searched CENTRAL (2014, Issue 6), MEDLINE (1950 to July week 3, 2014), EMBASE (1974 to July 2014), Web of Science (1900 to July 2014), the Chinese Biomedical Literature Database, which includes the China Biological Medicine Database (from 1978 to July 2014), the Chinese Medicine Popular Science Literature Database (from 2000 to July 2014) and the Masters Degree Dissertation of Beijing Union Medical College Database (from 1981 to July 2014). We also searched the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP) and ClinicalTrials.gov for completed and ongoing trials on 31 July 2014.Randomised controlled trials (RCTs) comparing probiotics with placebo to prevent acute URTIs.Two review authors independently assessed the eligibility and quality of trials, and extracted data using the standard methodological procedures expected by The Cochrane Collaboration.We included 13 RCTs, although we could only extract data to meta-analyze 12 trials, which involved 3720 participants including children, adults (aged around 40 years) and older people. We found that probiotics were better than placebo when measuring the number of participants experiencing episodes of acute URTI [at least one episode: odds ratio (OR): 0.53; 95% CI = 0.37-0.76, P < .001, low quality evidence; at least three episodes: OR: 0.53; 95% CI = 0.36-0.80, P = .002, low quality evidence]; the mean duration of an episode of acute URTI [mean difference (MD): -1.89; 95% CI = -2.03 to -1.75, P < .001, low quality evidence]; reduced antibiotic prescription rates for acute URTIs (OR: 0.65; 95% CI = 0.45-0.94, moderate quality evidence) and cold-related school absence (OR: 0.10; 95% CI = 0.02-0.47, very low quality evidence). Probiotics and placebo were similar when measuring the rate ratio of episodes of acute URTI (rate ratio: 0.83; 95% CI = 0.66-1.05, P = .12, very low quality evidence) and adverse events (OR: 0.88; 95% CI = 0.65-1.19, P = .40, low quality evidence). Side effects of probiotics were minor and gastrointestinal symptoms were the most common. We found that some subgroups had a high level of heterogeneity when we conducted pooled analyses and the evidence level was low or very low quality.Probiotics were better than placebo in reducing the number of participants experiencing episodes of acute URTI, the mean duration of an episode of acute URTI, antibiotic use and cold-related school absence. This indicates that probiotics may be more beneficial than placebo for preventing acute URTIs. However, the quality of the evidence was low or very low.