Experimental studies of nasal passage deposition were performed in three adult human subjects with ultrafine Po-218 aerosols to determine the inspiratory deposition efficiency of such aerosols. Constant flow rates in the range 5.9-17.51 min(-1) were employed with aerosols whose diameters ranged from 0.53 to 0.62 nm. The deposition efficiency was found to range from 93.7 to 98.7%, showing a weak dependence on flow rate. These results are compared to previously published nasal deposition measurements in replicate nasal airway casts and in human subjects and found to be in good agreement. It is concluded that the nasal passage is a highly efficient collector for unattached radon progeny aerosols whose particle diameter is of the order 1 nm, and that measurements in replicate nasal models accurately predict overall deposition at this size. These in vivo experiments extend the range of measured deposition of ultrafine aerosols to <1 nm.