Longitudinal studies have shown that preschool children's diagnosis of Oppositional Defiant Disorder (ODD), Conduct Disorder (CD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are likely to persist into school age. However, limited attention has been paid to instability of diagnosis. The aim of the present study, therefore, was to investigate both stability and change of ODD, CD and ADHD diagnosis in children aged 3.5-5.5 years. For diagnosing these disorders, a semi-structured diagnostic parent interview, i.e., the Kiddie-Disruptive Behavior Disorder Schedule (K-DBDS), was used at the first assessment and at follow-up assessments (9 and 18 months). Five diagnostic stability groups (chronic, partial remission, full remission, new onset, no diagnosis) were compared with regard to impairment and number of symptoms. Participants were referred preschool children with externalizing behavioral problems (N = 193; 83% male) and typically developing (TD) children (N = 58; 71% male). Follow-up assessments allowed to distinguish children belonging to the chronic group of ODD, CD or ADHD from those belonging to one of the remission groups. In addition, there was a substantial number of children with a new onset diagnosis. In conclusion, as a complement to studies showing stability of ODD, CD and ADHD diagnosis into school age, present findings point to changes of diagnosis in the preschool and early school period. Diagnostic reassessments therefore are needed in this age group.