Levels of PCDD/Fs and dioxin-like PCBs were measured in 16 pooled samples of serum from a total of 322 adults in the general population, to coincide with the start-up of a new municipal solid urban waste treatment plant in Biscay, Basque Country (Spain). Two hundred and eighty-three individual serum samples were also obtained, in which the most common PCBs (28, 52, 101, 118, 138, 153 and 180) were quantified. The samples were taken from four geographical zones: two from the metropolitan area of Bilbao, located less than 2 km from the plant and with high traffic density (Zones E1 and E2), a third located 5 km from the plant in an urban area of Bilbao, also with high traffic density (Zone C1) and the fourth located 20 km from the plant, in a municipality with minimal industrial activity and low traffic density (Zone C2), the latter two being out of the path of the prevailing winds. The median levels of dioxins+furans were similar by zone: E1 = 24.3, E2 = 27.3, C1 = 21.3, C2 = 18.8 pg g −1 lipid (p = 0.362); by sex: 20.2 vs. 22.6 pg g −1 lipid in men and women (p = 0.328); and by age: 20.8 vs. 21.3 pg g −1 lipid in subjects aged 20–44 and 45–69 ( p = 0.505). No detectable levels of PCBs 52 and 101 were found. Significant differences by zone were found only for PCB 180 ( p = 0.041), with higher values in Zone C2, the zone with the lowest presumed contamination levels. Dioxin-like PCBs ( p < 0.001) and the most common PCBs (138, 153, 180) ( p < 0.001) were both statistically associated with age, higher values being found in the 45–69 age group.