  Recent experimental results for neutron-neutron scattering length are
reanalyzed from the point of view of three-nucleon force contribution. We found
that the limiting value of $a_{nn}=- 15.8\pm 0.5$~fm must be free of any
implicit three-body force contribution. We have also shown that the difference
between the above experimental value of $a_{nn}$ and the well established value
of neutron-proton scattering length $a_{np}$ can be explained by differences in
the one-pion exchange potentials.
