BACKGROUND: An epidermal growth factor receptor (EGFR) immunohistochemical detection system currently is being developed. The current study attempts to address background EGFR reactivity issues before determining the optimum EGFR scoring system. METHODS: Tissue sections from 102 patients with T3N1-2M1 colon adenocarcinoma were stained with a prototype EGFR detection system. The number of cases, location, percentage, and intensity of reactive cells (0+ [none] to 3+ [strong]) were scored and compared with the length of survival. RESULTS: Approximately 75.5% of the adenocarcinoma cases had EGFR reactivity; 31.4% of the tumors had 3+ reactivity in 10-50% of the neoplastic cells and 3.9% had 3+ reactivity in > 50% of cells. Increased numbers of reactive cells per case predominantly resulted from increased 3+ reactivity. The mean percentage of 2+ (moderate) and 3+ reactive cells per case increased in the regions of deepest invasion. The mean percentage of 3+ reactivity per case was significantly greater in the deepest tumor region compared with the superficial region (16.9% vs. 7.9%; P = 0.004). EGFR reactivity in metastases appeared to have the strongest correlation with reactivity in the deep regions of colon adenocarcinoma. An increasing percentage of 2+ and 3+ or 3+ only reactivity in the deep region was found to have the strongest correlation with decreased survival (P = 0.0252). CONCLUSIONS: EGFR reactivity of 2+ and 3+ may provide a framework for a scoring system. It may be important to evaluate EGFR reactivity in the deepest region of tumor invasion because this region appears to contain the largest percentage of 3+ reactive cells and appears to have the strongest correlation with survival length and EGFR reactivity in lymph node and liver metastases.