This study examines the persistence of sleep problems over 18 months in 76 referred children with anxiety disorders and/or attention deficit hyperactivity disorders (ADHD) and 31 nonreferred controls, and explores predictors of sleep problems at follow-up (T2) in the referred children. Diagnoses were assessed at initial assessment (T1) using the semi-structured interview Kaufman Schedule for Affective Disorders and Schizophrenia. Sleep problems were assessed using the Children's Sleep Habit Questionnaire at T1 and at T2. Persistence rate of total sleep problems in the clinical range was 72.4 % in referred children, and did not differ significantly between children with a T1 diagnosis of anxiety disorder (76.0 %), ADHD (70.6 %), anxiety disorder and ADHD (68.8 %) or nonreferred controls (50.0 %) The total sleep problems score at T1 significantly predicted the total sleep problems score at T2, whereas age, sex, parent education level and total number of life events did not.