A 4-month-old Japanese girl with a developmental dislocation of the right hip was treated with manual reduction under general anesthesia followed by cast immobilization. Serial magnetic resonance imaging (MRI) revealed that a large interposition of fat tissue in the acetabulum, observed immediately after cast immobilization, became remarkably small in size as the congruity improved after 3 weeks. These MRI findings demonstrate the mechanism of a "squeezing phenomenon" after reduction of the hip.