To determine whether stabilizing serum glucose, via introduction of an insulin pump, improves classroom attention among children with type-1 diabetes mellitus. Four boys having type-1 diabetes mellitus with unstable serum glucose were observed in their classroom for 10 baseline days. An insulin pump was placed and serum glucose stabilized, and they were then observed again for 10 days. A modified multiple baseline design was used to determine if improved on-task and off-task behavior was associated with better glycemic control. Rating scales and a laboratory measure of attention, measures of secondary interest, were also administered before and after pump introduction, and potential improvement in individuals’ scores was evaluated. All boys had apparent improvement in on-task and off-task behavior as observed in their classrooms. Improvements were substantial, averaging 20% in on-task behavior and 34% in off-task behavior. However, no changes were detected on rating scales or laboratory measures. This study offers preliminary evidence that stabilizing serum glucose improves classroom attention, although the effect was detected only by observation of classroom behavior using highly structured techniques. Consequently, use of direct observation techniques may be important in studying the effects of chronic illness on classroom functioning.