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Incidences of inflammatory bowel disease (IBD) and of breast cancer have increased over the last decades. The influence of IBD on breast cancer prognosis, however, is unknown. We therefore examined the impact of IBD on treatment receipt and survival in breast cancer patients.Information on breast cancer patients (stage and treatment) diagnosed between 1980 and 2004 was sourced from the Danish Cancer Registry. Data on IBD and potential confounders were extracted from the Danish National Registry of Patients covering all Danish hospitals. Cox regression was used to compute mortality rate ratios (MRRs) among breast cancer patients with IBD, compared to their non-IBD counterparts, adjusting for age, stage, comorbidity measured by the Charlson Index, and calendar year.We identified 71,148 breast cancer cases; 67 also had Crohn's disease (CD) and 216 had ulcerative colitis (UC). Patients with CD had more advanced stage and received radiotherapy less, and chemotherapy more, frequently than patients without IBD. In the adjusted analyses there was no substantial survival difference in breast cancer patients with and without IBD (MRRCD = 1.22; 95% confidence interval [CI] = 0.85–1.75; MRRUC = 1.09; 95% CI = 0.86–1.38). In a stratified analysis, chemotherapy was associated with poorer survival in patients with CD (MRRCD = 1.93; 95% CI = 1.00–3.72).Breast cancer patients with UC receive the same treatment and have similar survival to breast cancer without IBD. In contrast, breast cancer patients with CD are treated with radiotherapy less often. Survival of breast cancer in patients with CD treated with chemotherapy is poorer compared to survival in patients without IBD.