Learning how to use a computer-based communication system can be challenging for people with severe aphasia even if the system is not word-based. This study explored cognitive and linguistic factors relative to how they affected individual patients' ability to communicate expressively using C-Speak Aphasia (CSA), an alternative communication computer program that is primarily picture-based. Ten individuals with severe non-fluent aphasia received at least six months of training with CSA. To assess carryover of training, untrained functional communication tasks (i.e., answering autobiographical questions, describing pictures, making telephone calls, describing a short video, and two writing tasks) were repeatedly probed in two conditions: (1) using CSA in addition to natural forms of communication, and (2) using only natural forms of communication, e.g., speaking, writing, gesturing, drawing. Four of the 10 participants communicated more information on selected probe tasks using CSA than they did without the computer. Response to treatment was also examined in relation to baseline measures of non-linguistic executive function skills, pictorial semantic abilities, and auditory comprehension. Only nonlinguistic executive function skills were significantly correlated with treatment response.