The aim of this study was to clarify the relationship between dioxin concentrations in breast milk and the sister chromatid exchange (SCE) frequency in women from herbicide-sprayed and non sprayed areas. Blood samples were taken from 21 women with high TCDD (tetrachlorodibenzo-p-dioxin) levels from sprayed areas, 23 women with moderate TCDD levels from sprayed areas, and 19 women from non sprayed areas to determine their SCE frequency. The SCE frequencies for the high and moderate TCDD groups from the sprayed area and for the non sprayed area group were 2.40, 2.19, and 1.48 per cell, respectively. Multiple regression analysis showed that the standardized beta values for 1,2,3,6,7,8-hexaCDD (beta = 0.60), 1,2,3,4,6,7,8-heptaCDD (beta = 0.64), and octaCDD (beta = 0.65) were higher than those for TCDD (beta = 0.34) and 1,2,3,7,8-pentaCDD (beta = 0.42). The adjusted R-2 value for polyCDDs (R-2 = 0.38) was higher than that for polyCDD toxic equivalents (TEQ (toxic equivalents); R-2 = 0.23). This study therefore shows that levels of hexa-, hepta-, and octaCDD, which were previously regarded as being less toxic than TCDD, are closely related to SCE frequency and that the level of dioxin (pg/g lipid) is potentially more useful as an indicator than TEQ value for explaining SCE frequency.