A randomized, double-blind, placebo-controlled clinical trial was conducted to determine whether oral administration of heat-killed Lactobacillus gasseri OLL2809 would affect the immune response and reduce the symptoms of Japanese cedar pollinosis (JCP) in subjects with JCP. Following a 1-week pre-observation period, the subjects were randomly divided into two groups and were orally administered a placebo or tablets containing 100 mg of L. gasseri OLL2809 per d for 8 weeks during the pollen season in 2007. The results showed no obvious differences between the groups. Supplementary subgroup analysis revealed that the OLL2809 subgroups with CAP-RAST scores of 4 or 5 exhibited improvement in nasal symptoms scores and serum allergy-related items, including Japanese cedar pollen-specific IgE levels. L. gasseri OLL2809 was found to be effective in reducing symptoms in subjects with a high predisposition to allergies by modulating systemic immune systems.