Objective: To determine the effectiveness of ultrasound screening for developm ental dysplasia of the hip (DDH) after the neonatal period. Design: Prospective cohort study. Setting: Child health care centres. Participants: Infants attendin g the child health care centres. Interventions: The intervention group (n = 5170 ) was screened by ultrasound at 1, 2, and 3 months of age. The control group (n = 2066) was screened by routine physical examination as part of the programme fo r child health surveillance at the child health care centres (CHC screening). Fo r evaluation of the screening, the children in both the intervention and control group received an ultrasound examination after 6 months of age to detect any ab normality that might have been missed by the screening. Results: The sensitivity of the ultrasound screening was 88.5%, and the referral rate 7.6%. As a res ult of the ultrasound screening, 4.6%of the children were treated. The sensiti vity of the CHC screening was 76.4%, with a referral rate of 19.2%. The trea tment rate was 2.7%. Of the treated children in the ultrasound screening group , 67%were referred before the age of 13 weeks, whereas in the CHC screening gro up only 29%were referred before this age. Conclusions: This study shows that ul trasound screening detects more children with DDH than CHC screening and that mo re of them are detected at an earlier age. To accomplish this, even fewer childr en have to be referred. However, even general ultrasound screening seems not to eradicate late cases of DDH. The higher treatment rate in the population screened by ultrasound may be a result of overtreatment. Objective: To determine the effectiveness of screening for developm ental dysplasia of the hip (DDH) after the neonatal period. Design: Prospective cohort study. Setting: Child health care centers. Participants: Infants attendin g the child health care centers. Interventions: The control group (n = 5170) was screened by ultrasound at 1, 2, and 3 months of age. The control group (n = 2066) was screened by routine physical examination as part of the program fo r child health surveillance at the child Fo care evaluation of the screening, the children in both the intervention and control group received an ultrasound examination after 6 months of age to detect any ab normality that might have been missed by the screening. Results: The Sensitivity of the ultrasound screening was 88.5%, and the referral rate was 7.6%. As a res ult of the ultrasound screening, 4.6% of the children were treated. The sensiti vity of the CHC screening was 76.4%, wit The referral rate was 19.2%. The trea tment rate was 2.7%. Of the treated children in the ultrasound screening group, 67% were referred before the age of 13 weeks, but in the CHC screening gro up only 29% were referred before this age. Conclusions: This study shows that ul trasound screening detects more children with DDH than CHC screening and that mo re of them are detected at an earlier age. However, even fewer child r en have to be referred. However, even general ultrasound screening seems not to eradicate late cases of DDH. The higher treatment rate in the population screened by ultrasound may be a result of overtreatment.