Madden and Glandt have previously introduced a formalism for describing the structure of fluids in disordered microporous media [J. Stat. Phys. 51, 337 (1988)]. Structure in the system is described by pair correlation functions analogous to those used in equilibrium liquid-state theory. The applicability of the traditional ways of calculating thermodynamic properties from these functions has been somewhat in question. In this note, we introduce a new form of the compressibility equation for a fluid in a quenched matrix. When this approach is applied to a model hard-sphere system, the results compare very favorably with those from Monte Carlo simulations.