The study uses an alternating treatment design to evaluate the functional effect of teacher's affect on students' task performance. Tradition in special education holds that teachers should engage students using positive and enthusiastic affect for task presentations and praise. To test this assumption, we compared two affective conditions. Three teachers working with a total of 4 elementary-age students with severe disabilities presented discrete trial tasks in neighborhood public school settings. The teachers alternated their affect between a positive affective condition characterized by smiling, enthusiasm, and rising and falling voice inflection and a neutral affect condition characterized by a flat voice tone and inexpressive facial features. Three of the four students responded more accurately to instructional tasks when teachers presented task demands and praise using positive affect. A fourth student, however, performed worse under the positive affect condition. This student had a long history of problem behavior in instructional settings. Implications for assessment practices are discussed. (Contains 2 figures.)