Attention Deficit Hyperactivity Disorder (ADHD) and dyslexia are frequently co-occurring disorders. Although methylphenidate (MPH) is the primary treatment for ADHD, the effect on reading in children with these comorbid problems is not yet known. This study was an unblinded clinical trial to evaluate the reading performance before and after treatment with MPH. Reading performance was compared with General Linear Model repeated measures between three groups: (1) an experimental group of children with both ADHD and dyslexia (N = 24), (2) a control group of children with ADHD (N = 9) and (3) a control group of children with dyslexia (N = 10). MPH improved reading performance significantly stronger in the experimental group than in the control groups; the number of correctly read words increased to a larger extent. In conclusion, MPH proved to be an aid in the reading process of children with ADHD and comorbid dyslexia by improving the learning conditions, but MPH cannot cure the reading disorder. Future research should study the effect of MPH on reading in a double-blind clinical trial.