We prove the ill-posedness of Leray solutions to the Cauchy problem for the hypodissipative Navier-Stokes equations, when the dissipative term is a fractional Laplacian with exponent . The proof follows the "convex integration methods" introduced by the second author and Laszl Sz,kelyhidi Jr. for the incompressible Euler equations. The methods yield indeed some conclusions even for exponents in the range [1/5, 1/2].