Purpose <p>This study aimed to evaluate the CT findings of lung adenocarcinoma with solid components and to determine the difference between adenocarcinoma in situ (AIS) and minimally invasive adenocarcinoma (MIA) with invasive adenocarcinoma (IAC).</p> Materials and methods <p>A total of 54 cases were included in this study. The diagnoses of lung adenocarcinoma consisted of AIS or MIA (<i>n</i> = 20) and IAC (<i>n</i> = 34). The following factors were evaluated on CT images: part-solid nodule or solid nodule, presence of air bronchogram, air space, calcification within the tumor, presence of interstitial pneumonia and emphysema, diameters of the tumor and solid component, and CT values of the solid component. The volume and CT number histograms, including the 50th, 75th, and 100th percentiles of solid component were obtained using a software program. The CT criteria were compared between AIS, MIA, and IAC, and an indicator of differentiation was considered.</p> Results <p>Part-solid nodules were observed more frequently in AIS and MIA (85.0%) than in IAC (55.9%). All criteria for quantitative analysis showed significant differences between AIS or MIA and IAC, and the diameter of the solid component in the mediastinal window was an indicator of differentiation (<i>p</i> = 0.0006; odds ratio, 1.4; 95% confidence interval, 1.2–1.8).</p> Conclusion <p>The diameter of the solid component on the mediastinal window was considered an indicator of differentiation between AIS, MIA, and IAC.</p> <p>Condensed abstract.</p> <p>Quantitative data of solid component, including both manual measurements and evaluation using CT software, are correlated with pathological invasiveness.</p> <p>Diameter of the solid component in the mediastinal window would be an indicator of IAC.</p>