  We determine the primordial helium mass fraction Yp using 93 spectra of 86
low-metallicity extragalactic HII regions. This sample constitutes the largest
and most homogeneous high-quality data sets in existence for the determination
of Yp. For comparison and to improve the statistics in our investigation of
systematic effects affecting the Yp determination, we have also considered a
sample of 271 low-metallicity HII regions selected from the DR5 of the SDSS.
Although this larger sample shows more scatter, it gives results that are
consistent at the 2sigma level with our original sample. We have considered
known systematic effects which may affect the 4He abundance determination. They
include different sets of HeI line emissivities and reddening laws, collisional
and fluorescent enhancements of HeI recombination lines, underlying HeI stellar
absorption lines, collisional excitation of hydrogen lines, temperature and
ionization structure of the HII region, and deviation of HeI and H emission
line intensities from case B. However, the most likely value of Yp depends on
the adopted set of HeI line emissivities. Using Monte Carlo methods to solve
simultaneously the above systematic effects we find a primordial helium mass
fraction Yp = 0.2472+/-0.0012 when using the HeI emissivities from Benjamin et
al. (1999, 2002) and 0.2516+/-0.0011 when using those from Porter et al.
(2005). The first value agrees well with the value given by SBBN theory, while
the value obtained with likely more accurate emissivities of Porter et al.
(2005) is higher at the 2sigma level. This latter value, if confirmed, would
imply slight deviations from SBBN.
