Epidermal growth factor (EGF) receptor expression was estimated in 50 invasive human colorectal cancers using immunohistochemistry and the degree of expression was quantified from integrated optical density measurements on the stained sections. All tumours stained positively, but Dukes' C tumours exhibited significantly higher levels of receptor than either Dukes' A or B tumours. In addition, histologically high grade cancers expressed receptors more strongly than those of low grade. It is concluded that a high EGF receptor concentration is associated with poor prognostic factors in colorectal malignancy.