BACKGROUND: Management of colorectal cancer depends on many patient and tumour variables. Decisions are sometimes based on the histological grade of tumour as assessed by initial biopsy. This retrospective study looked at the reliability of grading colorectal cancer from pre-operative biopsies. METHODS: 118 patients with a diagnosis of colorectal cancer, who had adequate pre-operative biopsies, were included in the study. A single consultant histopathologist re-examined all the preoperative biopsy slides, but was blinded to the final grading of the resection specimen. RESULTS: Of the 118 resection specimens, 41 were graded well-differentiated, 52 moderately differentiated and 25 poorly differentiated. When compared with postoperative grading, 37 (31%) initial biopsies were undergraded, 49 (43%) were correctly graded and 32 (26%) were overgraded. CONCLUSIONS: These results suggest that the histological grading of pre-operative specimens is inaccurate and should be used with caution to determine treatment. The difficulty with grading pre-operative biopsies in colorectal cancer may be owing to a lack of specific criteria such as is used in, for example, breast cancer.