Endocrine-disrupting compounds such as dioxin and PCBs have been hypothesized to influence sex ratio at birth. Births after the Seveso chemical spill that spewed dioxin over residential areas in Italy demonstrated a tendency towards fewer males. We studied sex ratio in births occurring 1964–1967 in the San Francisco Bay Area and measured specific PCB congeners using GC/ECD in maternal sera collected during the 2nd or 3rd trimester and stored at −20 degrees Celsius. The participants were a subset of those enrolled in the Child Health and Development Study, a cohort of about 20,000 pregnancies. As measurements were made on a selected sample of 399 who were followed for five years, subjects were assigned normalized weights according to the sampling fractions of males and females from the original cohort. Given the volume of serum available, the abundance of each CB, and the precision of the method, we report on eleven congeners: #101, #105, #110, #118, #137, #138, #153, #156, #170, #180, and #187. Additionally, we calculated a “total PCB” exposure based on those nine congeners with > 70% above the limit of detection. PCBs were expressed on a lipid basis (pg/g). A negative correlation was observed between the ratio of male:female offspring and maternal PCB concentration for each of the eleven congeners, though not all of these were significant at the 0.05 level. After adjustment for hormonal medications taken in the periconceptional and early pregnancy period, higher concentrations of total PCBs were significantly associated with a lower sex ratio (p = .01). In the highest quartile of total PCB exposure, the ratio was 42:58, or 0.72 (the expected sex ratio is about 1.05). A significant association was also seen for CB #170, and all congeners showed the same direction of association, an improbable finding under the null hypothesis. As PCB blood concentrations have been shown to remain very stable during pregnancy, these findings suggest that high maternal PCB concentrations, at least for some congeners, may either favor the fertilization by female sperm, or result in greater male embryonic or fetal losses.