BACKGROUND: Ultrasound (US) is routinely used for hip screening in children with developmental hip disorders, whereas standard hip surveillance in children with cerebral palsy is based on repeated X-ray assessments. ----- OBJECTIVE: To evaluate US as a diagnostic tool in screening for decentered hips in children with cerebral palsy. ----- MATERIALS AND METHODS: We conducted a prospective, diagnostic single-center assessor-blind study that included consecutive children (age 2-8 years) with cerebral palsy and severe motor disability who underwent US and X-ray hip assessment. US lateral longitudinal scans were used to determine lateral head distance. X-ray assessment was used to determine migration percentage. Diagnostic properties of lateral head distance in detecting hips with a migration percentage ≥0.33 (which requires preventive treatment) were evaluated overall (n = 100) and for hips assessed at the age 24-60 months (n = 38) or >60 to ≤96 months (n = 62). Fifty hips underwent US assessment by two investigators to evaluate inter-rater reliability and agreement. ----- RESULTS: Prevalence of migration percentage ≥0.33 was 22.0% overall and 26.2% and 19.4% in the younger and older age-based subsets, respectively. Lateral head distance well discriminated hips with a migration percentage ≥0.33 (areas under the receiver operating characteristics [ROC] curves 94%, 99% and 92%, respectively). At the optimum cut-off values of lateral head distance (5.0, 5.0 and 4.8 mm, respectively), sensitivity was 95.5%, 100% and 100% overall and in the two age-based subsets, respectively, whereas specificity was 85.9%, 96.4% and 72.0%, respectively. Consequently, positive predictive value was relatively low, but negative predictive value was 98.5% (95% CI 92.1-100) overall and 100% (97.5% one-sided CI 87.2-100) and 100% (97.5 one-sided CI 90.2-100) in the two age-based subsets, respectively. Inter-rater reliability was high (intraclass correlation coefficient = 0.98, 95% CI 0.97-0.99) and 95% limits of agreement were reasonably narrow (-1.203 mm to 0.995 mm). ----- CONCLUSION: In children with cerebral palsy, US can be reliably used in screening for decentered hips and can greatly reduce the need for repeated radiographic assessments, thus reducing radiation burden in these children.