Purpose: To assess the safety of unoprostone isopropyl 0.15% ophthalmic solution (UIOS) in patients with mild to moderate asthma. Methods: Randomized, double-masked, two-period crossover, placebo-controlled study comparing unoprostone 0.15% with vehicle-placebo. Pulmonary function tests conducted at baseline and during the 4-hour postdose observation period included forced expiratory volume in 1 s, forced vital capacity, peak expiratory flow rate, and bronchodilator response after 200 µg inhaled salbutamol. Results: There were no significant changes in pulmonary function up to 4 h postdose after treatment with 1 drop unoprostone 0.15% in each eye. Conclusion: Unoprostone 0.15% was safe for use in patients with mild to moderate asthma, without affecting the response to bronchodilator therapy.