We studied the outcome of ultrasound-monitored Pavlik harness treatment of 47 infants (nine boys and 38 girls) with 55 frankly dislocated hips documented by dynamic ultrasonography. The success and failure groups were of similar ultrasound status (hip-status score) at the time of initial examination, but a significant difference emerged at the 7- and 14-day examinations. Detection of frank dislocation and institution of treatment with the Pavlik harness within 3 weeks of birth increased the probability of a successful result. Of 33 infants (27 girls and six boys) with 41 dislocated hips who were seen before the age of 21 days, 26 (63%) hips were reducible in the Pavlik harness alone, and 15 (37%) hips were irreducible and required a subsequent procedure to achieve reduction. Fourteen infants (three boys and 11 girls) with 14 dislocated hips were first seen after the age of 21 days. Three (20%) were successfully treated in the Pavlik harness alone, and 11 (80%) required further treatment. In our opinion, persistent dislocation without improvement after 3 weeks of treatment in the Pavlik harness mandates discontinuation of use of this device. No anatomic factors were seen at the time of the initial ultrasound examination that permitted prediction of those hips likely to succeed or fail treatment with the Pavlik harness.