  We review experimental attempts using polarized neutron scattering technique
to reveal the existence in high temperature superconductors of a long-range
ordered state characterized by the spontaneous appearance of current loops. We
draw a particular attention to our recent results (B. Fauqu\'e {\it et al.},
{\it Phys. Rev. Lett.} {\bf 96}, 197001 (2006)) that, up to now, can be
explained only by the theory of circulating current proposed by C.M. Varma.
