  We consider the classical Cauchy problem for the 3d Navier-Stokes equation
with the initial vorticity $\omega_0$ concentrated on a circle, or more
generally, a linear combination of such data for circles with common axis of
symmetry. We show that natural approximations of the problem obtained by
smoothing the initial data satisfy good a-priori estimates which enable us to
conclude that the original problem with the singular initial distribution of
vorticity has a solution. We impose no restriction on the size of the initial
data.
