Background: Animal data demonstrate associations of dioxin, furan, and PCB exposures with altered male gonadal maturation. It is unclear whether these associations apply to human populations. Objectives: We investigated the association of dioxins, furans, PCBs and corresponding toxic equivalent (TEQ) concentrations with pubertal onset among boys in a dioxin-contaminated region. Methods: Between 2003-2005, 489 boys were enrolled at ages 8-9 years in a longitudinal study in Chapaevsk, Russia. Pubertal onset - stages 2 or higher for genitalia (G2+) or testicular volume (TV) > 3 ml - was assessed annually between ages 8-12 years. Serum levels at enrollment were analyzed by the Centers for Disease Control and Prevention, Atlanta, GA. Cox proportional hazards models were used to assess age at pubertal onset as a function of exposure adjusted for potential confounders. Sensitivity analyses were conducted excluding boys with pubertal onset at enrollment. Results: The median (range) total serum TEQ concentration was 21 (4-175) pg/g lipid, approximately three times higher than values in European children. At enrollment, boys were generally healthy and normal weight (mean BMI 15.9 kg/m2), with 30% having entered puberty by G2+ and 14% by TV criteria. Higher dioxin TEQs were associated with later pubertal onset by TV, hazard ratio = 0.68, 95% CI: 0.49-0.95 for the highest compared with the lowest quartile. Similar associations were observed for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and dioxin concentrations for TV but not G2+. Results were robust to sensitivity analyses. Conclusions: Findings support an association of higher peri-pubertal serum dioxin TEQs and concentrations with later male pubertal onset reflected in delayed testicular maturation.