A retinal prosthesis system to restore sight for the blind is under development. The system is analogous to cochlear implants, in which photoreceptor input is bypassed and replaced by direct electrical stimulation of the retinal ganglion cells. Currently, six test subjects have been implanted with a 4x4 electrode array and stimulator. We report here psychophysical clinical data examining how stimulation amplitude affects phosphene shape and repeatability on a single electrode. Phosphene shape data was quantified by a set of numerical descriptors calculated from image moments. Comparison of phosphene descriptors for a single electrode across repeated trials and amplitude levels measured the repeatability within an amplitude group. Our experimental findings show that stimulation of the retina creates repeatable percept shapes and that an increase in stimulation amplitude causes a significant change in size and shape of phosphenes.