OBJECTIVE: Developmental exposure to dioxin-like compounds has been associated with cognitive and motor impairments in children. These toxicants have been shown to be thyroid toxicants in animal studies. Therefore, the objective of this study was to quantify the overall dioxin-like activity in maternal serum and determine the association between dioxin-like activity and thyroid hormone levels. STUDY DESIGN: Cross-sectional examination of serum from pregnant women (n = 150) attending a prenatal diagnosis clinic between January 2002 and December 2003. RESULTS: Serum dioxin-like activity was measured in 145 of 150 (96.7%) maternal serum samples. The mean (+/- SEM) serum lipid-adjusted dioxin-like activity was 0.34 +/- 0.01 pg/g. Multiple regression analysis failed to demonstrate a relationship between maternal serum dioxin-like activity and serum thyroid hormone levels. CONCLUSION: Dioxin-like activity is quantifiable in an overwhelming majority of second-trimester maternal serum samples but there was no relationship between dioxin-like activity and thyroid hormone levels in our study population.