Three-loop QCD corrections to the vector current correlator
are considered. The large momentum procedure is applied in order to
evaluate mass corrections up to order $(m^2/q^2)^6$. The inclusion of
the first seven terms to the ratio
$R=\sigma(e^+e^-\to\mbox{hadrons})/\sigma(e^+e^-\to\mu^+\mu^-)$ leads
to reliable predictions from the high energy region down to relatively
close to threshold.