We develop a general formalism for including nonelectrostatic interactions, such as excluded volume interactions, in the Poisson-Boltzmann (PB) equation. The resulting theory can be applied to any boundary condition and is as easy to numerically implement as the original PB equation. As a specific example, we combine the PB equation with the Boublik-Mansoori-Carnahan-Starling equation of state to model charged hard-sphere systems. This theory is applied to a charged sphere immersed in a salt solution, and the electric field and ion distribution about the sphere are computed.