This study systematically examined the functional relation between screaming and environmental events in a 13-year-old boy diagnosed with autism and PDD-NOS. A functional analysis of screaming was conducted in study 1, with six conditions (attention, demand, play, alone, tangible—toy, and tangible—edible). Highest rates of screaming were observed during the tangible—toy and tangible—edible conditions. The analog assessment was followed by a descriptive assessment, which revealed the participant was most likely to scream when teacher attention was diverted to other students. Additionally, items found to maintain screaming during the analog assessment were never delivered contingent upon screaming in the participant's natural environment. The results of the descriptive assessment suggested that the analog assessment might have produced a false-positive outcome. A treatment based upon the results of the combined assessments led to a 50% reduction in screaming within four weeks. Two-year follow-up data indicated that screaming occurred at near-zero rates. Copyright © 2003 John Wiley & Sons, Ltd.