Sediment accumulation of organic halogen was studied in two forest lakes, one pristine and one which received 30 m(3) of biologically purified bleaching wastewater from a kraft pulp mill in 1979 equivalent to ca. 2 kg of adsorbable organic halogen (AOX). Lake sediments were dated with(210)Pb,(134)Cs and(137)Cs and the annual deposition rates of organic halogens and organic matter were calculated. Organic bound halogen contents of the sediment aged 150 years was 180 microg Cl g(-1) d.w. in both lakes. The concentration of organic bound halogen at the topmost 6 cm of the sediments (less than 20-years-old) ranged from 45 to 80 microg Cl g(-1) d.w. This suggests that solvent extractable halogen had enriched in the older sediment layers. The deposition of extractable organic halogen (EOX) in the lakes in 1950's was 4 to 5 mg Cl m(-2) a(-1). Since then, the deposition of EOX doubled in both lakes. The deposition of organic matter increased concomitantly from 50 g m(-2) a(-1) to 110 g m(-2) a(-1) in Lake Mustalampi and from 35 g m(-2) a(-1) to 62 g m(-2) a(-1) in Lake Pyylampi suggesting that the increase in the deposition of organic halogen followed the increase in the deposition of organic matter. Of the 2 kg of organic halogen discharged into the lake, 5% or less was detected in the sediment in tetrahydrofuran extractable form 15 years later.