We quantitatively investigated nanoparticle deposition (density: 1.0 g/cm(3), diameter: 0.1-1.0 mu m) in a pulmonary acinus using computational fluid dynamics. We assumed that acinar flow was induced by the expansion and contraction of the acinar model, with the volume changing sinusoidally with time as the boundary condition. We based the complicated acinar model and volume change on a mammalian lung, and evaluated the distribution of particle depositions through acinar generation based on three dimensional thinning. The maximum Reynolds numbers at the inlet and terminal alveoli were 0.191 and 1.93 x 10(-3), respectively. With only gravitational force, particle deposition was strongly affected, except for the 0.1- and 0.2-mu m particles, and the 0.8- and 1.0-mu m particles were deposited around the inlet due to sedimentation after the beginning of inflation. We found that the 0.1- to 0.4-mu m particles were deposited at the distal alveoli and that the deposition was enhanced by both the gravitational and Brownian forces, despite the small terminal velocity. For the 0.1- and 0.2-mu m particles in particular, the particles were deposited throughout the acinus. The maximum deposition fraction of each acinar segment reached 1.2% after a single breath, and was higher in the middle generations and at the terminal alveoli. (C) 2016 Elsevier Ltd. All rights reserved.