  Four of the six parameters defining the two-loop \pi\pi scattering amplitude
have been determined using Roy dispersion relations. Combining this information
with the Standard \chi PT expressions, we obtain the threshold parameters,
low-energy phases and the O(p^4) constants $l_1^r, l_2^r$. The result $l_2^r (
M_{\rho} ) = ( 1.6 \pm 0.4 \pm 0.9 ) \times 10^{-3} ( {\bar l_2} = 4.17 \pm
0.19 \pm 0.43$) reproduces the correct D-waves but it is incompatible with
existing Standard \chi PT analyses of $K_{l4}$ form factors beyond one loop.
