The aim of this study was to determine drug-dependent effects on attention and motor activity in children with attention-deficit/hyperactivity disorder (ADHD) in a double-blind methylphenidate (MPH) trial using both subjective behavior ratings and objective measures of function. In particular, we were interested in the relationship between changes of those subjective and objective measures. A total of 44 ADHD children with the combined subtype completed a randomized, double-blind, placebo-controlled, crossover trial with 2 doses of MPH (0.25; 0.5 mg/kg body weight) and placebo. In each condition, computerized attention tasks were performed, teacher-behavior ratings were obtained, and actigraph data were collected in both clinical and naturalistic settings. Trend tests revealed linear effects of MPH dose on actigraph data in the test session (p = 0.02) and at school (p = 0.001), as well as on sustained attention (p < 0.001), while inhibitory control showed a quadratic dose-response curve (p < 0.001). Multivariate regression analyses revealed that changes of both, hyperactive-impulsive symptoms (28%) and inattentive symptoms (23%) could be explained by objective changes of motor activity. Thus, for clinical practice, it should be taken into account that behavior ratings of ADHD symptoms seemed to be predominantly influenced by changes of motor activity.