  An analytic collapse model for the formation and density distribution of
virialized cold dark matter halos is proposed. Hierarchical structure formation
is taken into account explicitly. Monte Carlo methods are used to generate
samples of mass histories of virialized halos. The mean density distribution
found from the collapse model is in good agreement with numerical results in
the mass range from $10^{11}M_\odot$ to $10^{15}M_\odot$ and in the radial
range form $0.05 r_{200}$ to $r_{200}$.
