2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has a high estimated cancer potency in animals which has been reasoned to imply that TCDD might be carcinogenic to man. The animal cancer data show that TCDD can act in a solitary manner causing tumors without the participation of other known factors. However, there exist animal cancer data indicating that TCDD can act as a tumor-promoting compound. The analysis examines which type of carcinogen and which mechanism best characterize TCDD cancer activity. It is suggested that TCDD acts by a hormonal mechanism to cause cancer in a solitary manner, at low doses, in two species, and in a number of different organs, including rare sites. These observations in toto characterize TCDD as a complete carcinogen, which by definition encompasses both initiation and promotion carcinogenic activities.