OBJECTIVES: To examine whether rate of reoperation after breast conserving surgery is associated with patients' characteristics and investigate whether reoperation rates vary among English NHS trusts. DESIGN: Cohort study using patient level data from hospital episode statistics. SETTING: English NHS trusts. PARTICIPANTS: Adult women who had breast conserving surgery between 1 April 2005 and 31 March 2008. MAIN OUTCOME MEASURE: Reoperation rates after primary breast conserving surgery within 3 months, adjusted using logistic regression for tumour type, age, comorbidity, and socioeconomic deprivation. Tumours were grouped by whether a carcinoma in situ component was coded at the time of the primary breast conserving surgery. RESULTS: 55,297 women had primary breast conserving surgery in 156 NHS trusts during the three year period. 11,032 (20.0%, 95% confidence interval 19.6% to 20.3%) women had at least one reoperation. 10,212 (18.5%, 18.2% to 18.8%) had one reoperation only; of these, 5943 (10.7%, 10.5% to 11.0%) had another breast conserving procedure and 4269 (7.7%, 7.5% to 7.9%) had a mastectomy. Of the 45,793 women with isolated invasive disease, 8229 (18.0%) had at least one reoperation. In comparison, 2803 (29.5%) of the 9504 women with carcinoma in situ had at least one reoperation (adjusted odds ratio 1.9, 95% confidence interval 1.8 to 2.0). Substantial differences were found in the adjusted reoperation rates among the NHS trusts (10th and 90th centiles 12.2% and 30.2%). CONCLUSION: One in five women who had breast conserving surgery in England had a reoperation. Reoperation was nearly twice as likely when the tumour had a carcinoma in situ component coded. Women should be informed of this reoperation risk when deciding on the type of surgical treatment of their breast cancer.