The compound 2,3,7,8-tetrachlorodibenzo-p-dioxin, commonly known as dioxin, was measured in the adipose tissue of 39 persons with a history of residential, recreational, or occupational exposure in Missouri and in 57 persons in a control group. All participants had detectable levels of 2,3,7,8-tetrachlorodibenzo-p-dioxin in their adipose tissue, but the exposed group had significantly higher levels. Levels of six of the exposed persons were more than five times greater than the level of the highest control. Measuring 2,3,7,8-tetrachlorodibenzo-p-dioxin in adipose tissue provides a much improved index of exposure, an important advance for research studies evaluating the possible health effects of this compound.