Cystic Fibrosis (CF) lung disease is characterized by identifiable structural abnormalities on CT or UTE MRI, such as mucus plugs and bronchiectasis. Similarly, hyperpolarized 129Xe MRI can provide functional information related to regional pulmonary ventilation, with high sensitivity to early obstruction. Here we combined UTE and 129Xe MRI to quantify the extent to which specific structural abnormalities contribute to regional ventilation defects. Mucus plugs and bronchiectasis account for the majority of ventilation defects that can be attributed to structural abnormalities. However, structurally-unattributable ventilation defects are more common, implying higher sensitivity than structural imaging to lung dysfunction in milder disease.