Purpose. Sonoporation is an ultrasound technique that enables large molecules that normally do not penetrate the cell membrane to pass through it. Recent studies show that pulsed ultrasound in the presence of microbubbles increases the permeability of the cell membrane. However, the mechanism and basic properties of this sonoporation remain unclear. We thus investigated the mechanism of generation and frequency of occurrence of sonoporation, as well as the repair of a cell membrane damaged by microbubbles. Methods. The spatial relationship between microbubbles and cells was observed microscopically when cells were sonicated with pulsed ultrasound. Effects of microbubbles on the cells were observed with a high-speed camera, and the ratio of cell membrane damage and repair was examined using fluorescent microscopy. Results. Damage to the cell membrane, caused mainly by mechanical effects of the expansion and contraction of microbubbles, significantly increased the permeability of the cell membrane. The frequency of cell membrane damage was closely associated with the presence of microbubbles and increased with increase in acoustic pressure. The ratio of repair of damaged cells was about 70% during 3 min after a single shot of pulsed ultrasound, indicating that repair of damaged cell membranes requires little time. Conclusion. We examined the frequency of occurrence of cell membrane damage and repair in sonoporation using pulsed ultrasound and microbubbles. Our results should prove useful for improving pulsed-ultrasound sonoporation.