In our recent survey, fern spores were found to be the third most common airborne air spora in the Bangkok atmosphere; therefore, the significance of fern spores as a causative allergen was assessed in patients with allergic rhinitis. By using the extract from the spores of Acrostichum aureum Linn, which is a widespread fern, an intracutaneous test was done in 226 allergic rhinitis patients and 61.5% of these patients reacted positively. Nasal provocation test by using the same kind of fern spore extract was also performed in 24 allergic rhinitis patients and the positive reaction occurred in 70.8%. When skin tests and nasal provocation tests were performed in a group of 20 nonallergic ENT patients, only 20% gave weakly positive skin test reactions and 15% gave mild reactions to the nasal provocation test. The authors concluded that fern spores are potentially allergenic and allergists should be aware that ferns could be an important aeroallergen.