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Background: Insomnia disorder in childhood and adolescence has severe implications on overall well-being and development. Age-specific treatments for insomnia disorder with cognitive behavioral interventions (CBT-I) are available and effective. Nightmare disorder also has severe consequences in children and adolescents. However, less is known about children with insomnia (I) and comorbid nightmare disorder (I + N). Methods: In this retrospective study, data from 499 children and adolescents with insomnia disorder were included. The prevalence of a comorbid nightmare disorder (I + N) was calculated within three subsamples (toddlers and preschoolers 0.5–4 years, elementary school children 5–10 years, and adolescents 11–18 years). Differences between children with insomnia (I) and those with additional nightmare disorder (I + N) regarding age, sex, family background, sleep quality (SOL, WASO, TST, and SE) based on sleep logs, behavior sleep problems (based on interviews), and behavioral problems (CBCL and YSR) were calculated within each age group. Results: The overall prevalence of additional nightmares or nightmare disorder in children or adolescents with insomnia was 15–24%. We found various clinically relevant differences between I and I + N for each age group; for example, there were more sleep onset association problems in I + N elementary school children, prolonged SOL of 56 min, and about 50 min less TST and SE of 76.8% in I + N adolescents. However, most statistical tests were not significant. Especially sleep parameters but also emotional burden were more pronounced in I + N groups than in the I groups. Toddlers and preschoolers with I + N were significantly older than those with only I, had another family situation (e.g., divorced parents) significantly more often, and I + N adolescents were statistically more often anxious and depressed. Discussion: Descriptively, I + N children and adolescents seemed to be more impaired than those with insomnia only. However, a comorbid nightmare disorder cannot be recognized by insomnia-specific sleep parameters. Therefore, diagnostic procedures for insomnia should always screen for nightmares but also other sleep disorders. If necessary, CBT-I should be supplemented with nightmare-specific interventions.