  We aim to study the structure and kinematics of the broad line region (BLR)
of a sample of 27 gravitationally lensed quasars with up to five different
epochs of observation. This sample is composed of ~100 spectra from the
literature plus 22 unpublished spectra of 11 systems. We measure the magnitude
differences in the broad emission line (BEL) wings and statistically model the
distribution of microlensing magnifications to determine a maximum likelihood
estimate for the sizes of the C IV, C III], and Mg II emitting regions. The
BELs in lensed quasars are expected to be magnified differently owing to the
different sizes of the regions from which they originate. Focusing on the most
common BELs in our spectra (C IV, C III], and Mg II), we find that the
low-ionization line Mg II is only weakly affected by microlensing. In contrast,
the high-ionization line C IV shows strong microlensing in some cases,
indicating that its emission region is more compact. Thus, the BEL profiles are
deformed differently depending on the geometry and kinematics of the
corresponding emitting region. We detect microlensing in either the blue or the
red wing (or in both wings with different amplitudes) of C IV in more than 50%
of the systems and find outstanding asymmetries in the wings of QSO 0957+561,
SDSS J1004+4112, SDSS J1206+4332, and SDSS J1339+1310. This observation
indicates that the BLR is, in general, not spherically symmetric and supports
the existence of two regions in the BLR, one insensitive to microlensing and
another that only shows up when it is magnified by microlensing.
