Lahore has been enveloped by intense fog for a period of 1 to 2 months during the previous three winters. The extensive nature of the fog has caused severe disruption to transport and the economy in Northeastern Pakistan. In this paper we report aerosol data from a field study carried out from 25 December 1999 to 8 January 2000 during the fog period. High pollutant concentrations were observed throughout the field study with SO2 from 0.3 to 24.7 ppb, SO42- from 4 to 141 mug/m(3), and NO3- from 3 to 74.5 mug/m(3). Exceptionally high trace element aerosol concentrations were also observed, for example, Se concentrations up to 258 ng/m(3), As of 26 ng/m(3), and Sb of 84.8 ng/m(3). Pb concentrations up to several mug/m(3) were observed well above; the WHO guideline of 1 mug/m(3). Maximum aerosol concentrations were observed during the daytime compared to nighttime which is consistent with increased urban and photochemical activity throughout the day. Source apportionment revealed four major groupings. Group one is comprised of the crustal elements Al, Mg, Fe, Co, Cr, Mn, Ca, and Ni and is associated with airborne soil material. The second grouping contained Pb, Sb, As, Zn, and Cu due to vehicular and various industrial processes. Approximately 70% of the Pb was ascribed to leaded gasoline. Group three contained SO42-, NO3-, and Ni which were ascribed to coal and oil combustion sources. The final group contained Se, As, and Sb with Se the highest loading suggesting a local source, probably industrial. The very high aerosol concentrations observed in Lahore pose a serious health risk in a highly populated urban center (population of approximately 7 million). An in-depth study of the fog chemistry and aerosol size measurements are required in order to develop appropriate control strategies and to assess possible impacts on climate and human health.