Abstract: A method and an agent for improving the retention when dewatering a cellulosic fiber suspension are disclosed In the method, there is added to the suspension a retention agent containing a phenol-formaldehyde resin and a polyethylene oxide, and the method is characterised in that a modified phenol-formaldehyde resin containing cationic groups is added to the suspension. The modified phenol-formaldehyde resin preferably has a cationicity of about 5-85% and the cationic groups preferably consist of amine groups. The polyethylene oxide in the retention agent preferably has a molecular weight of about 0.5-20×106. The modified phenol-formaldehyde resin and the polyethylene oxide are preferably added to the suspension in a weight ratio of modified phenol-formaldehyde resin:polyethylene oxide of from 1:5 to 50:1, and the amount of polyethylene oxide added preferably is about 10-1000 ppm, based on tie dry solids content of the suspension.