In recent years, there has been a debate on whether a considerable fraction of particles is retained after 24 h in the tracheobronchial region. In the present study, 8 healthy subjects inhaled 6.2-microns monodisperse Teflon particles labeled with 111 In twice, at flow rates of 0.45 and 0.045 L/s. According to theoretical calculations, the particles inhaled at 0.45 L/s should deposit mainly in large bronchi and in the alveolar region, whereas the particles inhaled at 0.045 L/s should be deposited mainly in small ciliated airways. Twenty-four hours after inhalation, about half of the particles inhaled with both modes of inhalation had cleared. Clearance during the period from 1 to about 30 days after inhalation, could, for both modes of inhalation, be described by the sum of two exponential functions. For the inhalation rate of 0.45 L/s, 15% cleared with a half-time of 3.4 days and 85% with a half-time of 190 days. For the inhalation of 0.045 L/s, 20% cleared with a half-time of 2.0 days and 80% with a half-time of 50 days. The results strongly indicate (1) that a considerable fraction of particles deposited in small ciliated airways had not cleared within 24 h, and (2) that these particles cleared differently from particles deposited in the alveolar region. The experimental data agree quite well with the IRTM predictions made using its default slow clearance fractions.