Wood ash produced as a by-product in paper mills and heating power plants contains many toxic heavy metals, including high cadmium (Cd) concentrations. The addition of wood ash on drained and forested peatlands to improve forest growth and nutrient balance is common practice in Finland but creates a risk of increasing Cd con-centration in the soil and Cd release into recipient waters. Leaching of Cd from ash-treated peat was studied in a mesocosm experiment with three different ash doses (300, 600 and 1,200 g ash m -2), which have been commonly applied for fertilization in boreal forests and forested peat-lands. Ash contained 5.4 mg Cd kg -1 , which corresponds doses of 1.6, 3.2 and 6.5 mg Cd m -2 . The mesocosm studies showed that all ash treatments clearly lowered pH (from 3.6 to 3.4–3.3) and dissolved organic carbon concentrations (from 97 to ca. 50 mg C l -1) in the run-off water during an 11-week experiment. However, only the application of the highest dose, 1,200 g ash m -2 , increased the Cd concen-trations in the run-off water from the initial concentration of 1.25–1.75 lg Cd l -1 . The treatment of peat with 300 or 600 g ash m -2 did not increase Cd leaching as compared to the untreated peat. We measured high pH values (between 6.1 and 7.0) and high Cd concentrations only in the upper layers of ash-treated peat, which clearly indicated that most of Cd was tightly retained in the surface peat. The effects of Cd on the aquatic ecosystem were studied in microcosms where run-off water from mesocosms was added into aquariums containing humic lake water, lake sediment, bryophytes and benthic animals. The microcosm experiment showed that only the run-off water received from the highest ash treatment increased the Cd concentration of microcosm water. Greater Cd concentrations were found in benthic animals, Asellus aquaticus isopod, in treatments with the highest ash doses, but accumulation of Cd was not observed in the bottom material or in the bryophytes (Fontinalis an-tipyretica). According to the results, the risk of Cd release from peat after ash application is of minor short-term importance when low ash doses (between 300 and 600 g ash m -2) are used. Ash addition, however, tempo-rally increased the acidity of discharge water, which may contribute to the mobilization of Cd from the peat and enhance the risk of bioaccumulation in aquatic organisms especially in acid waters.