Nasal delivery of drugs and vaccines has important advantages compared to injection and oral administration, and is being considered for a widening range of vaccines and substances with topical and systemic action. Traditional nasal delivery technologies are, however, trapped in the dilemma between achieving improved nasal distribution and limiting deposition in the lower airways. The novel bi-directional nasal delivery concept takes advantage of the posterior connection between the nasal passages persisting when the soft palate automatically closes during oral exhalation. Exhalation into the delivery device triggers release of liquid or powder particles into an airflow, which enters one nostril via a sealing nozzle and exits through the other nostril. In a study of 16 healthy subjects using 99mTc labeled nebulized particles with a mean particle size of 3.5 microm, delivery with this novel concept showed no or minimal lung deposition (0.8 +/- 2.0% (range -4.1% to 5.6%) for bi-directional delivery, whereas significant fractions were deposited in the lungs in all 16 subjects (mean 22.3 +/- 8.1%, range 12.2-39.3%) following conventional nasal inhalation (p < 0.0005). In the latter case, the fraction deposited in the lungs correlated significantly (r2 = 0.47, p < 0.004) with the volume of the nasal passages. The bi-directional nasal delivery concept minimizes the risks and problems related to lung deposition occurring during conventional nasal inhalation from a nebulizer and opens up a new range of opportunities for nasal delivery of drugs and vaccines.