  We develop a theory of magnetoresistance of two-dimensional electron systems
in a smooth disorder potential in the hydrodynamic regime. Our theory applies
to two-dimensional semiconductor structures with strongly correlated carriers
when the mean free path due to electron-electron collisions is sufficiently
short. The dominant contribution to magnetoresistance arises from the
modification of the flow pattern by the Lorentz force, rather than the magnetic
field dependence of the kinetic coefficients of the electron liquid. The
resulting magnetoresistance is positive and quadratic at weak fields. Although
the resistivity is governed by both viscosity and thermal conductivity of the
electron fluid, the magnetoresistance is controlled by the viscosity only. This
enables extraction of viscosity of the electron liquid from magnetotransport
measurements.
