In Experiment 1, 28 attention-deficit hyperactivity disorder (ADHD) boys underwent a double-blind, placebo-controlled medication assessment in a summer day-treatment program. Daily, boys were asked questions to assess their attributions for and evaluations of their behavior. Objective measures showed improved behavior with methylphenidate; however, boys tended to attribute their performance to effort rather than to medication, particularly when medicated. Experiment 2 involved 38 ADHD boys the following summer and replicated the procedures in Experiment 1, with the addition of a no-pill condition and a comparison of attributions for success and failure outcomes. Simply taking a pill (no-pill vs. placebo comparison) did not show significant effects, whereas the results of Experiment 1 were replicated with placebo-methylphenidate comparisons. Across drug conditions a self-enhancing attributional pattern was obtained; the majority of attributions for success were to ability or effort, whereas attributions for failure were to the pill or to counselors.