The ion atmosphere created by monovalent (Na+) or divalent (Mg2+) cations surrounding a B-form DNA duplex were examined using atomistic molecular dynamics (MD) simulations and the nonlinear Poisson-Boltzmann (PB) equation. The ion distributions predicted by the two methods were compared using plots of radial and two-dimensional cation concentrations and by calculating the total number of cations and net solution charge surrounding the DNA. Na+ ion distributions near the DNA were more diffuse in PB calculations than in corresponding MD simulations, with PB calculations predicting lower concentrations near DNA groove sites and phosphate groups and a higher concentration in the region between these locations. Other than this difference, the Na+ distributions generated by the two methods largely agreed, as both predicted similar locations of high Na+ concentration and nearly identical values of the number of cations and the net solution charge at all distances from the DNA. In contrast, there was greater disagreement between the two methods for Mg2+ cation concentration profiles, as both the locations and magnitudes of peaks in Mg2+ concentration were different. Despite experimental and simulation observations that Mg2+ typically maintains its first solvation shell when interacting with nucleic acids, modeling Mg2+ as an unsolvated ion during PB calculations improved the agreement of the Mg2+ ion atmosphere predicted by the two methods and allowed for values of the number of bound ions and net solution charge surrounding the DNA from PB calculations that approached the values observed in MD simulations. © 2014 Wiley Periodicals, Inc. Biopolymers 101: 834–848, 2014.