Understanding the filtration efficiency of the nasal and oral airways is essential for assessing doses of inhaled particles to the extrathoracic region as well as to the lung. This paper presents in vivo measurements of nasal airway dimensions and the extrathoracic deposition of ultrafine aerosols in 10 normal adult males. The nasal geometry of each subject was characterized using magnetic resonance imaging and acoustic rhinometry. The nasal and oral deposition efficiencies were measured for particles ranging from 4 to 150 nm at constant flow rates of 167 and 333 cm(3) s(-1). Results indicated that both nasal dimensions and particle deposition varied significantly among individuals. Inter-individual variability in particle deposition was correlated with the wide inter-individual variation of nasal dimensions measured by the total surface area, minimum cross-sectional area, and complexity of the airway shape. We concluded that the significant biological variability in extrathoracic filtration of ultrafine aerosols must be considered in developing population-wide dosimetry of inhaled particles.