Adaptive SPH and N-body simulations were carried out to study the evolution of the equilibrium structure of dark matter halos that result from the gravitational instability and fragmentation of cosmological pancakes. Such halos resemble those formed by hierarchical clustering from realistic initial conditions in a CDM universe and, therefore, serve as a test-bed model for studying halo dynamics. The dark matter density profile is close to the universal halo profile identified previously from N-body simulations of structure formation in CDM, with a total mass and concentration parameter which grow linearly with scale factora. When gas is included, this concentration parameter is slightly larger than the pure N-body result. We also find that the dark matter velocity distribution is less isotropic and more radial than found by N-body simulations of CDM. ©2001 American Institute of Physics.