  A reanalysis is made for the helium abundance determination for the
Izotov-Thuan (2004) spectroscopic sample of extragalactic H II regions. We find
that the effect of underlying stellar absorption of the He I lines, which is
more important for metal poor systems, affects significantly the inferred
primordial helium abundance $Y_p$ obtained in the zero metallicity limit and
the slope of linear extrapolation, $dY/dZ$. This brings $Y_p$ from
$0.234\pm0.004$ to $0.250\pm 0.004$ and $dY/dZ=4.7\pm 1.0$ to $1.1\pm 1.4$.
Conservatively, this indicates the importance of the proper understanding of
underlying stellar absorption for accurate determinations of the primordial
helium abundance to the error of $\delta Y_p\simeq 0.002-0.004$.
