The presence of epidermal-growth-factor (EGF) receptors in normal and neoplastic human urothelium was studied in 12 control patients and in 48 patients with transitional cell carcinoma of the bladder, 24 with invasive (pT3) and 24 with superficial tumours (9 pT1, 15 pTa). EGF receptors were identified on frozen sections by means of an indirect immunoperoxidase technique with a monoclonal antibody against the EGF receptor. Significantly more invasive tumours (21 of 24) than superficial (7 of 24) were stained positively for the EGF receptor (X2 = 14.49; p less than 0.001). Significantly more poorly differentiated tumours (18 of 21) than moderately differentiated tumours (10 of 27) were EGF-receptor positive (X2 = 9.6; p less than 0.01). No control sample stained positively for the EGF receptor. These findings suggest that the presence of a high intensity of staining for the EGF receptor in human bladder tumours is associated with poor differentiation and with invasion.