Seven pulmonary adenocarcinomas with cloudy zone (CZ), which was defined as a lower density zone in the pulmonary lesion than pulmonary vessels in the surrounding normal lung on high resolution computed tomography (HRCT), were followed up for at least 3 months. There were three tumors with homogeneous CZ, one of which showed no change when rescanned after three months, the others showed enlargement with or without replacement by high density areas on rescanned images obtained after more than one year. Two tumors containing some high density areas on the initial scans showed slight decreases in size on rescanned images obtained after 1 to 3 months. However, on rescanned images obtained after more than 1 year and 9 months, these tumors also showed enlargement of CZ. Pathologically, CZs corresponded to well-differentiated adenocarcinoma of the bronchioloalveolar type with no or mild interstitial change, whereas high density areas mainly corresponded to papillary adenocarcinoma with severe interstitial fibrosis. In conclusion, long-term careful follow-up of CZ is necessary to detect enlargement or the appearance of high density. Unchanged CZ in short-term follow-up should be considered a finding suggestive of malignancy.