A study of background polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzo-p-dioxins (PCDDs) in office buildings was conducted. Two multistory office buildings in Boston, Massachusetts were studied. Both were constructed in the 1960s and had never had an electrical transformer fire or failure. Twenty eight surface wipe and 16 air samples were collected and analyzed for PCDDs, PCDFs, or polychlorinated biphenyls (PCBs). Total surface PCDD concentrations ranged from 2.8 to 146 nanograms per square meter (ng/m2) and total surface PCDF concentrations from 0.47 to 5.4ng/m2. Octachlorodibenzo-p-dioxin (3268879) (OCDD) accounted for 86 percent of the PCDDs present. The PCDF isomers were more uniformly distributed. 2,3,7,8-Tetrachlorodibenzofuran (51207319) was present in 19 wipe samples at concentrations of 0.06 to 0.40ng/m2. Twenty three of 24 wipe samples contained PCBs at concentrations ranging up to 40 picograms per m2. The PCB concentrations were significantly correlated with the PCDF and PCDD concentrations. PCDD and PCDF concentrations on the surfaces of air handling units ranged from 69 to 421 and 6.5 to 53ng/m2, respectively. Airborne PCDFs were detected in only three samples, at a maximum concentration 1.4 picograms per cubic meter. The distribution of PCDD congeners in the air samples was similar to that of the wipe samples. The authors conclude that PCDFs and PCDDs are found as background air contaminants in the two office buildings, PCDDs being present at higher concentration. When making contamination assessments of PCDDs and PCDFs in office buildings their background concentrations should be considered.