In conventional mammography, x rays transmitted through the breast are converted to light in a phosphor screen, and the light exposes a film emulsion. The information in the image is degraded in this detector due to limitations in the screen and film. Photodiode arrays can convert the x rays directly into charge and overcome these problems. A preliminary investigation of a thick crystalline silicon photodiode array as a solid state digital detector was performed. The prototype device consists of a 300 micrometers thick, 256 X 256 photodiode array of 30 X 30 micrometers 2 pixels. The array was hybridized to two different readout structures for evaluation purposes, one structure being used for imaging and the other for single pixel experiments. Imaging performance, such as linearity, resolution, and noise were measured and used to predict the performance of a proposed clinical version of the prototype. Results show the detector response to be linear over the range of exposures required for mammography, the modulation transfer function (MTF) to be superior to that of screen-film detectors, and the noise to be dominated by x-ray quantum fluctuation. Based on results from the prototype devices, we predict that the detective quantum efficiency (DQE) of the clinical design will be significantly higher than that of a screen-film detector for all spatial frequencies of interest.