The concentrations of the enviromental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin were measured in the blood of 44 infertile women with endometriosis (study group), and in 35 age-matched women with tubal infertility (control group). Eight women with endometriosis (18%) were dioxin positive as compared to one woman (3%) in the controls (P = 0.04). Although the concentrations of dioxin did not seem to be directly correlated with the severity of endometriosis, these observations contribute to the accumulating data linking dioxin to endometriosis in humans.