Most healthy young people can use a visual P300-based speller in a controlled laboratory setting1. Previous studies in people with severe disabilities have been limited to relatively small populations [2,3,4] or have been in populations with a variety of different disorders [5,6]. Here, we evaluate the performance of a larger population of people severely disabled by ALS. We sought to identify factors that affect performance and methods for improving performance. The study is part of a program that is providing the P300 speller for long-term independent home use.