The Cologne Adaptive Multimodal Treatment (CAMT) study demonstrated that adaptive and individually tailored multimodal treatment for attention-deficit/hyperactivity disorder (ADHD) [consisting of behavior therapy (BT) and/or stimulant medication] is highly effective. This study reports findings of the 18 month follow-up assessment. Parents and teachers completed broad range behavior scales (Child Behavior Checklist/Teacher Report Form) and standardized ADHD and oppositional defiant disorder/conduct disorder symptom rating scales. Children that used medication to treat ADHD at follow-up (N = 32) and those that did not (N = 34) were analyzed separately. Parents did not report significant changes in child behavior from posttest to follow-up. Teacher ratings revealed some aggravation of ADHD symptoms in children that received medication, but this was not significant after Bonferroni correction. The initial advantage of combined treatment over BT was no longer evident. It can be concluded that treatment for ADHD that is tailored to the assessed needs of children results in large treatment effects that are maintained for at least 18 months.