  We consider for the first time the ability of present-day cosmic microwave
background (CMB) anisotropies data to determine the primordial helium mass
fraction, Y_p. We find that CMB data alone gives the confidence interval 0.160
< Y_p < 0.501 (at 68% c.l.). We analyse the impact on the baryon abundance as
measured by CMB and discuss the implications for big bang nucleosynthesis. We
identify and discuss correlations between the helium mass fraction and both the
redshift of reionization and the spectral index. We forecast the precision of
future CMB observations, and find that Planck alone will measure Y_p with
error-bars of 5%. We point out that the uncertainty in the determination of the
helium fraction will have to be taken into account in order to correctly
estimate the baryon density from Planck-quality CMB data.
