Ultrafine particles (< 0.10 microm in diameter) are present in great number in polluted urban air, thus posing a potential health risk. In this study, the total deposition fraction (TDF) of ultrafine aerosols with a narrow size distribution (number median diameter NMD = 0.04-0.1 microm and geometric standard deviation sigma(g) = approximately 1.3) was measured in a group of young healthy adults (11 men and 11 women). TDF was obtained with 6 different breathing patterns: tidal volume (V(t)) of 500 ml at respiratory flow rates (Q) of 150 and 250 ml/s; V(t) = 750 ml at Q of 250 and 375 ml/s; and V(t) = 1 L at Q of 250 and 500 ml/s. Aerosols were monitored continuously by a modified condensation nuclei counter while subjects were inhaling them with prescribed breathing patterns. For a given breathing pattern, TDF increased as particle size decreased, regardless of the breathing pattern used. For example, with V(t) = 500 ml and Q = 250 ml/s, TDF (mean +/- SD) was 0.26 +/-.04, 0.30 +/-. 05, 0.35 +/-.05, and 0.44 +/-.07 for NMD = 0.10, 0.08, 0.06, and 0. 04 microm, respectively. For a given NMD, TDF increased with an increase in V(t) and a decrease in Q. TDF was greater for women than men at NMD = 0.04 microm within all breathing patterns used (p <.05), but the difference was smaller or negligible for larger sized particles. The results suggest that the TDF of ultrafine particles increases with a decrease of particle size and with breathing patterns of longer respiratory time, a pattern that is consistent with diffusion deposition of ultrafine particles. The results also suggest that there is a differential lung dose of ultrafine particles and thus there may be a differential health risk for men versus women.