Asthma is a chronic obstructive lung disease characterized by recurrent episodes of bronchoconstriction and wheezing. Conventional asthma treatment involves the suppression of airway inflammation or improving airway flow. , also known as rosemary, is a Mediterranean plant that is used for the treatment of inflammatory diseases. Carnosol, a diterpenoid found in rosemary extracts, has been known to exhibit anti-inflammatory, anti-tumor, and anti-oxidant effects. The effect of carnosol on allergic responses has not been tested yet. The effect of carnosol on a murine allergic asthma model were investigated. Carnosol inhibited the degranulation of RBL-2H3 mast cells. Carnosol treatment inhibited the increase in the number of eosinophils in the bronchoalveolar lavage fluids (BALF) of mice treated with ovalbumin. Carnosol treatment also inhibited inflammatory responses and mucin production in histologic studies. Carnosol treatment inhibited the increases of IL-4 and IL-13 cytokines expression in both BALF and the lungs. These results suggest that carnosol may have a potential for allergic asthma therapy.