OBJECTIVE: We conducted a blinded, randomized, controlled trial to examine whether mildly dysplastic but stable or instable hips would benefit from early treatment, as compared with watchful waiting. PATIENTS AND METHODS: A total of 128 newborns with mild hip dysplasia (sonographic inclination angle [α angle] of 43°–49°) and stable or instable but not dislocatable hips were randomly assigned to receive either 6 weeks of abduction treatment (immediate-treatment group) or follow-up alone (active-sonographic-surveillance group). The main outcome measurement was the acetabular inclination angle, measured by radiograph, at 1 year of age. RESULTS: Both groups included 64 newborns, and there was no loss to follow-up. With the exception of a small but statistically significant excess of girls in the active-sonographic-surveillance group, there were no statistically significant differences in baseline characteristics between the 2 groups. The mean inclination angle at 12 months was 24.2° for both groups (difference: 0.1 [95% confidence interval (CI): −0.8 to 0.9]), and all children had improved and were without treatment. The mean α angle was 59.7° in the treatment group and 57.1° in the active-surveillance group for a difference of 2.6° evaluated after 1.5 and 3 months (95% CI: 1.8 to 3.4; P < .001). At 1.5 months of age, the hips had improved in all treated children but not in 5 children under active surveillance (P = .06). Among the sonographic-surveillance group, 47% received treatment after the initial surveillance period of 1.5 months. CONCLUSIONS: Active-sonographic-surveillance halved the number of children requiring treatment, did not increase the duration of treatment, and yielded similar results at 1-year follow-up. Given a reported prevalence of 1.3% for mildly dysplastic but stable hips, a strategy of active surveillance would reduce the overall treatment rate by 0.6%. Our results may have important implications for families as well as for health care costs.