We have developed a formalism for extending mean-field theories of isotropic fluids to systems containing anisotropic interactions. Our approach is a generalization of a method previously used by Flapper and Vertogen [Phys. Rev A 24, 2089 (1981) and J. Chem. Phys. 75, 3599 (1981)]. We illustrate our formalism by extending the van der Waals theory for isotropic fluids to anisotropic fluids with a nematic phase, and obtain results which are similar to those of Gelbart and Baron [J. Chem. Phys. 66, 207 (1976)] and Cotter [in The Molecular Physics of --- Liquid Crystals, edited by G. R. Luckhurst and G. W. Gray (Academic, London, 1979), p. 181]. The theory contains two dimensionless parameters which give measures of the anisotropy of attractive and repulsive parts of the intermolecular interactions. In the present paper we examine the temperature dependence of the order parameter and density in the low-pressure limit. An advantage of the present approach is that it offers a way of generalizing a wide class of equations of state to anisotropic fluids, and we therefore expect the method to be useful in systematically providing empirical equations of state for mesophase systems. Requirements of thermodynamic consistency are automatically satisfied.