This study examined (a) demographic and clinical characteristics associated with sleep-related problems (SRPs) among youth with anxiety disorders, and (b) the impact of anxiety treatment: cognitive-behavioral therapy (CBT; Coping Cat), medication (sertraline), their combination, and pill placebo on SRPs. Youth (N = 488, ages 7-17, 50% female, 79% White) with a principal diagnosis of generalized anxiety disorder, separation anxiety disorder, or social phobia participated. SRPs were reported by parents and youth. Findings differed by informant and by type of SRP, with evidence that SRPs are associated with age, anxiety severity, externalizing problems, functional impairment, and family burden at pretreatment. Anxiety treatment reduced SRPs; effect sizes were small to medium. Reductions in parent-reported separation-related sleep difficulties were significantly greater in active treatment than in the placebo condition, with the greatest reductions reported by parents of youth whose active treatment was multimodal or included sertraline. Youth whose anxiety treatment involved CBT reported significantly greater decreases in dysregulated sleep (e.g., sleeplessness). Both CBT for anxiety and sertraline appear to be somewhat effective in reducing SRPs, and multimodal treatment may be preferable depending on the symptom presentation. To inform practice, future research should examine a broad range of SRPs, incorporate objective measures of sleep, and evaluate the impact of behavioral strategies that directly target SRPs in youth with anxiety disorders.