To model sprays from pressure-swirl atomizers, the connection between the injector and the downstream spray must be considered. A new model for pressure-swirl atomizers is presented which assumes little knowledge of the internal details of the injector, but instead uses available observations of external spray characteristics. First, a correlation for the exit velocity at the injector exit is used to define the liquid film thickness. Next, the film must be modeled as it becomes a thin, liquid sheet and breaks up, forming ligaments and droplets. A linearized instability analysis of the breakup of a viscous, liquid sheet is used as part of the spray boundary condition. The spray angle is estimated from spray photographs and patternator data. A mass averaged spray angle is calculated from the patternator data and used in some of the calculations. This new model is referred to as the Linearized Instability Sheet Atomization (LISA) model, and is a complete set of equations which provide the required boundary data for the multi-dimensional, transient code, KIVA-3V. KIVA-3V is used to model the development of the spray, including spray collision, coalescence, breakup, and drag. The calculated results are compared to experimental data for two injectors. The spray penetration is compared to measured values from the literature. The spray calculations are also compared qualitatively to photographs of sprays. Further comparisons to transient patternator data are used to check the predicted mass flux distribution.