We discuss the effects of microlensing on the broad emission lines (BELs) of QSOs in the light of recent determinations of the size of the broad-line region (BLR) and its scaling with QSO luminosity. Microlensing by star-sized objects can produce significant amplifications in the BEL of some multiple-imaged QSOs, and could be very relevant for high-ionization lines. We have identified a group of 10 gravitational lens systems (~30% of the selected sample) in which microlensing could be observed. Using standard kinematic models for active galactic nuclei, we have studied the changes induced in the line profile by a microlens located at different positions with respect to the center of the BLR. We found that microlensing could produce important effects such as the relative enhancement of different parts of the line profile or the displacement of the peak of the line. The study of BEL profiles of different ionization in a microlensed QSO image could be an alternative method for probing the BLR structure and size.