Ductal carcinoma in situ (intraductal carcinoma) of the breast is a commonly recognized and curable clinical entity. Patients with intraductal carcinoma are at risk to develop invasive breast cancer presumably due to a transition from the noninvasive to the invasive phase of growth. Primary breast malignancies commonly display both in situ and invasive phases of growth in the same tumor. In the current study, DNA content and alterations in the erbB-2 (HER-2/neu) oncogene product were examined simultaneously in both growth phases of primary breast cancers by image analysis. DNA content in the intraductal and invasive components of primary breast cancers were virtually identical (r = 0.979, p < 0.001). Quantitative image analysis was used to measure erbB-2 expression and categories of expression were related to copy number of the erbB-2 gene. Expression of erbB-2 was similar in both growth phases and implies identity of the erbB-2 genotype. The identity of DNA content suggests that the noninvasive and invasive phases within a single breast cancer are highly related. It is likely that erbB-2 gene number remains the same during progression from intraductal to invasive disease.