In Denmark, the sludge produced in fish farms is usually applied as agricultural fertiliser. However, in some areas of Denmark, the cadmium and nickel contents of the fish farm sludge exceed the Danish quality criteria for sludge, which means that the sludge has to be deposited or remedied until it meets the criteria. Phytoremediation by willow may combine accumulation of cadmium and nickel from the sludge with the production of an energy crop. The ability of eight selected willow clones to take up and tolerate cadmium and nickel was studied in pots under outdoor conditions. Fish farm sludge was added to the pots at different dosages, either on the soil surface, mixed into the soil or as pure sludge. The remediation potential of the single clones was estimated after one growing season by multiplying growth and heavy metal content. At leaf-fall leaves were collected for cadmium and nickel measurements. After leaf-fall, the aboveground biomass was harvested as well as some of the roots. The remediation potential was larger when the willow grew in pure sludge compared to sludge in combination with soil. By extrapolating the results to a normal cultivation system with repeated wood harvest, the removal of cadmium and nickel from the sludge in every harvest cycle was calculated. On basis of the experiments it seems likely that the remediation method may be optimised so that the cadmium content can be lowered sufficiently after a 20-year period, if fish farm sludge is applied to soil at low dosages. For nickel it seems totally unrealistic to clean fish farm sludge by growing willow, unless the nickel remediation potential is increased, e.g. by chemically increasing nickel bioavailability in combination with breeding/selection of more potent clones or species.