"Picture Exchange Communication System" (PECS) training was implemented with 5 nonspeaking adults with mental retardation who were not currently using any type of functional communication system. A modified ABAB, single-subject design was used to assess the effectiveness of PECS in enhancing the functional communication skills of these individuals. Three individuals progressed through 4 PECS training phases relatively quickly and developed functional skills that they were able to display in home and community settings. Two other individuals demonstrated limited progress, and the PECS training did not meaningfully alter their level of communicative competence. Implications for teaching functional communication skills to nonspeaking adults are discussed, and recommendations for future research are provided.