A population-based study was conducted including all women diagnosed in British Columbia in 1991 with invasive node negative breast cancer (n = 942) in order to identify factors associated with variation in use of breast conserving surgery (BCS) and to determine if provincial practice guidelines were followed. Patient, disease, treatment and physician-specific information was abstracted from medical records and original source documents. 413 (44%) patients received BCS (51% and 23% in surgical candidates and non-candidates, respectively). Significant independent factors associated with BCS included patients' age, residence, family income, tumour size, tumour location, and extent of ductal carcinoma in-situ. Age and income had a significant interaction with stronger income effects in older women. A strong surgeon effect was observed which was not explained by measured surgeon attributes. Expansion of radiation treatment facilities may help address access issues. Further examination of the patient-physician relationship and of ways to assist patients in decision making is needed.