Behavioral sleep problems are common in children with attention-deficit/hyperactivity disorder (ADHD), yet their persistence or otherwise is unknown. We examined behavioral sleep problem trajectories, types of sleep problems experienced, and associated risk/protective factors.Design: Prospective cohort study. Setting: Twenty-one pediatric practices across Victoria, Australia. Participants: A total of 195 children with ADHD (5–13 years). Outcomes: Sleep problem trajectories classified as never, transient, or persistent on the basis of sleep problem severity measured at baseline, 6, and 12 months. Explanatory variables: Types of sleep problems, internalizing and externalizing comorbidities, ADHD symptom severity and medication use, autism spectrum disorder, caregiver mental health, and sociodemographic factors. Analyses: Multinomial logistic regression models.Sleep problems fluctuated over 12 months, but for 10% of children they persisted. In adjusted analyses, co-occurring internalizing and externalizing comorbidities were a risk factor for persistent (odds ratio (OR) 9.2, 95% confidence interval (CI) 1.6, 53.9, p = 0.01) and transient (OR 3.7, 95% CI 1.5, 8.8, p = 0.003) sleep problems, while greater ADHD symptom severity and poorer caregiver mental health were risk factors for persistent and transient sleep problems, respectively.Sleep problems in children with ADHD are commonly transient, but in a subgroup they are characterized as persistent. Early preventive/intervention strategies should target children at risk of persistent sleep problems.