Co-firing tests with coarse sawdust and Polish coal have been carried out at FORTUM's Naantali-3 CHP power plant (315 MWfuel). Naantali-3 plant is a tangentially fired pulverised coal unit with a Sulzer flow-through boiler that produces 79 MW electricity 124 MW district heat and 70 MW steam. Naantali-3 is equipped with roller coal mills (Loesche), modern low-NOx-burners (IVO RI-JET), over fire air (OFA), electrostatic precipitator (ESP) and flue gas desulphurization plant (FGD). Coal and sawdust were blended in the coal yard, and the mixture was fed into the boiler through coal mills. Performance tests were being conducted over a period of three weeks. A total of 670 tonnes (as received, 38-51% moisture) of pine and spruce sawdust from the sawmill were burned. The first test week was used to define, in the technical sense, the maximum proportion of wood in the blend when the coal/wood waste blend was introduced to one coal mill. Based on these experiments, the coal/sawdust mixture was introduced to all coal mills. During the actual co-firing tests, the proportion of sawdust in the blend varied between 4 and 10 per cent by mass (1 to 4 per cent from the fuel input). The co-firing tests were successful in many ways, but the behaviour of the coal mills caused some problems, and therefore the simultaneous feed might not be the solution in a long-term use.