Purpose: To analyze margin characteristics of pulmonary nodules on highresolution CT (HRCT) in order to improve imaging diagnoses. Material and Methods: HRCT images of 193 pulmonary nodules of less than 30 mm maximum diameter (113 primary cancers, 15 metastatic cancers, 55 inflammatory nodules, and 10 benign tumors) were reviewed and classified as to 6 types of margins: round, lobulated, densely spiculated, ragged, tentacle or polygonal and halo. the relationships of these imaging types to the diagnoses, the underlying pathological features, mainly those of tumor growth patterns in 93 neoplasms, and the pathological characteristics of 14 inflammatory nodules were investigated. Results: Eighty-two percent of the lobulated, 97% of the densely spiculated, 93% of the ragged and 100% of the halo nodules were malignant. Eighty percent of the tentacle or polygonal nodules were inflammatory and 66% of the round ones were benign. the 6 types differed statistically as to the nature of the benignity/malignancy ( p<0.001). Pathologically, in case of neoplasms, most of the 6 types had a relationship to a particular tumor growth pattern. Conclusion: This HRCT classification method is useful for determining the nature of small pulmonary nodules and reflects the underlying pathological characteristics.