The present study investigated the relationships among three approaches to the assessment of activity (laboratory measures, teacher ratings, and classroom observations) in a sample of second- and third-grade boys (n = 40) from regular classrooms. Results showed that only one of four laboratory measures (the stabilimeter score during a structured task) showed a significant correlation with observed motor activity in the classroom (r = .52, p < .05, n = 17), while none correlated reliably with teacher ratings. Total scores, using the hyperactivity factor of the Conners Teacher Rating Scale, correlated reliably with the amount of observed motor activity (r = .55, p < .05, n = 17), but not with the proportion of off-task intervals (r = .11, ns). Analysis of individual rating items indicated that ratings of excitability were reliably correlated with observed classroom activity (r = .49, p < .05, n = 17); however, ratings of “restless and over-active” were not correlated with observed classroom activity (r = .35, ns). Results are discussed with respect to the situation specificity of activity and the influence of direction of behavior versus amount of motor activity on teacher ratings of hyperactivity.