  Anisotropic hydrodynamics is a reorganization of the relativistic
hydrodynamics expansion, with the leading order already containing substantial
momentum-space anisotropies. The latter are a cause of concern in the
traditional viscous hydrodynamics, since large momentum anisotropies generated
in ultrarelativistic heavy-ion collisions are not consistent with the
hypothesis of small deviations from an isotropic background, i.e., from the
local equilibrium distribution.
  We discuss the leading order of the expansion, presenting a new formulation
for the (1+1)--dimensional case, namely, for the longitudinally boost invariant
and cylindrically symmetric flow. This new approach is consistent with the well
established framework of Israel and Stewart in the close to equilibrium limit
(where we expect viscous hydrodynamics to work well). If we consider the
(0+1)--dimensional case, that is, transversally homogeneous and longitudinally
boost invariant flow, {the new form of anisotropic hydrodynamics leads to
better agreement with known solutions} of the Boltzmann equation than the
previous formulations, especially when we consider finite mass particles.
