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This study was designed to develop a mathematical model for predicting the number of lymph nodes harvested in bowel cancer resection specimens based on the current clinical practice in the United Kingdom.Prospective clinical data were collected from 8,409 newly diagnosed bowel cancer patients presenting to 79 hospitals in Great Britain and Ireland during a variable 12-month period from 2000 to 2002. A two-level hierarchical regression model was used to identify predictors for lymph node harvest. The model was internally validated by comparing observed and model predicted lymph node harvest for patient subgroups.Inclusion criteria were satisfied by 5,164 patients. The average lymph node harvest was 11.7 nodes with significant between-center variability in lymph node harvest (range, 5.5-21.3 nodes). Increasing age, American Society of Anesthesiology grade, and preoperative radiotherapy were associated with a reduction of lymph node harvest (P< 0.001). Abdominoperineal resection of the rectum and transverse colectomy were the lowest yield procedures for lymph node harvest. Independent predictors of lymph node harvest were age, American Society of Anesthesiology grade, Dukes stage, operative urgency, type of resection, and preoperative radiotherapy. When tested, the model was found to accurately predict lymph node harvest for group statistics (comparison of observed and model predicted lymph node harvest F1,5154 = 0.63;P= 0.427).The results of the study suggest that the minimum number of lymph nodes harvested in colorectal cancer surgery cannot be set at a fixed value. The lymph node harvest model provides a simple tool to the frontline clinician for comparing standards between multidisciplinary bowel cancer teams.