Potentially, one of the most useful warning signals for the detection of environmental hazards is an unusually early age at onset of disease. Animal studies have shown age effects of this type with co-carcinogens. A clearcut example of a downward age shift in humans is provided by a study of the consumption of alcoholic beverages and cigarettes in women with oral concer. Using data on 145 white females with intraoral cancer, and 1973 non-neoplastic controls from patients seen at Roswell Park Memorial Institute between 1957 and 1966, it can be shown that exposure to both alcohol and tobacco can lead to onset of oral cancer 15 or more years earlier than would occur in women who do not use either alcohol or tobacco. Exposure to smoking only produces a smaller age shift, but exposure to alcohol only does not produce any clear shift in age of onset. Implications for co-carcinogenesis and for early detection of co-carcinogens in the environment are suggested.