A depth-averaged, convection-dispersion equation is derived for the particle volume fraction distribution in the pressure-driven flow of a concentrated suspension of neutrally buoyant, non-colloidal particles between two parallel plates, by implementing a two-time-scale perturbation expansion of the suspension balance model (Nott & Brady, J. Fluid Mech., vol. 275, 1994, pp. 157–199) coupled with the constitutive equations of Zarraga et al. (J. Rheol., vol. 52, issue 2, 2000, pp. 185–220). The Taylor-dispersion coefficient in the macrotransport equation scales as $U_c^{\prime}B^{\prime 3}/a^{\prime 2}$, where $U_c^{\prime}$ is the characteristic depth-averaged velocity, $B^{\prime}$ is the half-depth of the channel and $a^{\prime}$ is the particle radius. Taylor dispersion relaxes gradients in the depth-averaged volume fraction along the local velocity vector. Perpendicular to the flow, however, only shear-induced migration can cause particle redistribution, leading to fluxes down gradients in volume fraction, shear rate and streamline curvature that scale as $U_c^{\prime}a^{\prime 2}/B^{\prime}$. To determine the velocity and particle distributions in Hele-Shaw suspension flows, one only needs to solve two coupled partial differential equations in the pressure and the depth-averaged volume fraction, achievable on commercially available solvers. Analogous to the macrotransport equation for suspension flow through a circular tube (Ramachandran, J. Fluid Mech., vol. 734, 2013, pp. 219–252), the evolution of the particle volume fraction distribution is dependent only on the total strain experienced by the suspension, and is independent of the suspension velocity scale. However, unlike the tube problem, a positive concentration gradient along the flow direction is susceptible to viscous miscible fingering. A linear stability analysis performed for a step increase in the volume fraction in the direction of flow with a velocity $U^{\prime}$ reveals that the growth rate and wavenumber corresponding to fastest growing mode scale as $U^{\prime}a^{\prime 2}/B^{\prime 3}$ and $a^{\prime 2/3}/B^{\prime 5/3}$, respectively.