The fate of fuel nitrogen in the furnace of a full-scale bubbling fluidized bed boiler was studied in an extensive measurement campaign. The study of the fate of the fuel nitrogen is based on measurements in the furnace of a 107 MWth bubbling fluidized bed boiler for 3 days, each day with a different fuel blend. During the first day, the boiler was fired with bark. During the second day, the boiler was fired with a mixture of bark and sludge. During the third day, bark, sludge, and solid recovered fuel (SRF) were co-combusted. The furnace gas composition was measured from the fuel inlet to the furnace exit level, at seven in-furnace measurement ports in total. The main nitrogen species in the lower furnace, under the fuel inlet, were NH3 and NO, but also some HCN was found. Under reducing conditions in the lower furnace, nitrogen was mainly in the form of NH3, while for oxidizing conditions, the main nitrogen species measured were NO and NH3. The highest NH3 concentrations were observed above the fuel inlet. A total of 78–96% of fuel-N was measured as NH3, NO, and HCN above the fuel inlet. Moving above the secondary air level, a drastic reduction in these nitrogen species was observed. The main nitrogen species between the secondary and tertiary air levels was NH3. At the furnace exit, most NH3 and HCN had converted to NO and N2. When sludge was a part of the fuel mixture, the emssions of NO were higher but still well under the emission limits. A good reduction of nitrogen was observed for all of the cases. The fuel-N conversions to stack NO for the three cases were 6% for the bark and bark + sludge cases, and 7% for the bark + sludge + SRF case.