Selection of patients for breast-conserving surgery relies on inexact parameters such as the preoperative estimation of lesion size. This study investigates the value of needle core biopsy findings, in particular, the relative quantity of DCIS, in improving patient selection for breast conservation. Patients undergoing breast-conserving surgery for invasive ductal carcinoma from 1999 to 2004 were identified. Only patients who had a preoperative diagnosis of carcinoma (DCIS and invasive) on core biopsy were included. All core biopsies were reviewed by a breast histopathologist to document the quantity and characteristics of the DCIS component. Of a total of 281 patients, 46% (n=129) had invasive disease on core biopsy (group 1) and 54% (n=152) had either invasive disease with an accompanying DCIS component or DCIS only on core biopsy (group 2). The compromised margin rate for group 1 was 23% compared to 39% for group 2 (P=0.004). The rate of compromised margins increased progressively as the core biopsy DCIS component increased until a rate of 75% (n=18/24) was reached in patients with DCIS only on core biopsy. In patients with a DCIS component on core biopsy, the presence of necrosis (P=0.002), solid type architecture (P=0.008), high grade DCIS (P=0.007), calcification (P=0.003), and the relative proportion of DCIS present (P<0.001) were associated with compromised margins on univariate analysis. On multivariate analysis of this subgroup, the proportion of DCIS in this subgroup (P=0.048) was an independent predictor of compromised margins. The presence and relative proportion of DCIS on core biopsy provides important information as to whether patients are at risk of compromised margins. Documentation of these parameters may assist patient selection for breast-conserving surgery or identify patients who may benefit from wider margins at the time of initial operation.