Non-specific effects of methylphenidate treatment, including expectancy and regression to the mean effects, contribute to the overall effect of methylphenidate on attention-deficit/hyperactivity disorder (ADHD) symptoms. Knowledge on the extent to which non-specific effects contribute to the overall effect and whether regression to the mean explains part of the non-specific effects, is currently lacking. A double-blind, randomized, placebo-controlled, cross-over trial was used to compare parent and teacher ratings of child ADHD symptoms at baseline and during treatment with placebo and 5, 10, 15 and 20 mg of methylphenidate, twice daily. Participants were 5-13-year-old children with a DSM-5 diagnosis of ADHD (N = 45). The extent to which non-specific effects contributed to the effects of methylphenidate was determined by ADHD symptom reductions observed with placebo versus reductions observed with active doses of methylphenidate. The influence of regression to the mean was examined by estimating the contribution of baseline ADHD symptom severity to the effects observed with placebo treatment. Data were analyzed using multilevel analyses. We observed significant non-specific effects of methylphenidate for parent-rated ADHD symptoms, but not for teacher-rated symptoms. For parent reported hyperactive/impulsive symptoms, higher baseline symptoms predicted larger effects with placebo, indicating regression to the mean effects. For parent-reports, a significant part of the overall effect of methylphenidate treatment is explained by non-specific effects. Our findings stress the importance of taking non-specific effects into account when evaluating methylphenidate treatment, by including teacher-reports and using a double baseline assessment during titration. Comparing active medication with a placebo in the titration trial has the potential to identify non-specific effects.