The North Trent Cancer Network Breast Group in the United Kingdom revised its adjuvant treatment guidelines in breast cancer management in 1998. We aimed to check the compliance to the guidelines, 8 months after their introduction. Data were collected, retrospectively, from the medical records of patients with invasive breast cancer who underwent definitive surgery (in a 3-month period) in different cancer units and the cancer centre within the North Trent Cancer Network. The overall compliance to treatment guidelines was 82% (90% and 74% for chemotherapy and radiotherapy, respectively), which was similar across the network. In 5% of cases, compliance could not be determined. On case review, 22% of the non-compliant incidents were justified and 16% seemed to be due to variation in guideline interpretation. We discuss the possible reasons for non-compliance and show a need to periodically monitor compliance to adjuvant treatment guidelines.