The evidence supporting the prophylactic treatment of seasonal allergic rhinitis before the start of pollen dispersal is still lacking. We conducted a study to investigate the efficacy of prophylaxis with montelukast for seasonal allergic rhinitis and to evaluate its influence on the inflammatory condition of the lower airway. Our final study population was made up of 57 adults who were randomized to a prophylactic treatment group and a control group. The prophylaxis group was made up of 31 patients-10 men and 21 women, aged 18 to 54 years (mean: 36.9)-who were administered montelukast for 2 weeks before the cypress pollen season and subsequently throughout the remainder of the season. The control group was made up of 26 patients-11 men and 15 women, aged 24 to 63 years (mean: 39.2)-who took montelukast during the pollen season only. During the pollen season, the mean daily rescue medication score was significantly lower in the prophylaxis group (3.22 vs. 3.89; p = 0.001). However, there was no statistical difference in the two groups' mean daily rhinoconjunctivitis symptom scores. Also, the fraction of exhaled nitric oxide in the prophylaxis group tended to be lower than that of control group, but again the difference was not significant (29.8 vs. 42.1 ppb; p = 0.189). We conclude that antileukotriene prophylaxis started 2 weeks before the cypress pollen dispersal was effective in reducing the need for rescue medication during the pollen season and showed a trend toward alleviating the eosinophilic inflammation in the lower airway induced by the pollen.