A semi-physical auto-regressive moving average model of bending stiffness of the board produced at Assi Doman-Frovifors Bruk AB, as a function of measured control and disturbance variables, was identified. Based on the bending stiffness model, an adaptive on-line bending stiffness index predictor was implemented and found to have an RMS-error within the laboratory measurement accuracy. The predictor has been running for several months and consistently has a prediction accuracy of more than 75% within +/- one standard deviation, for all product grades. The main difficulty in developing the predictor was that bending stiffness is seldom measured. The predictor will form the basis of a model predictive regulator of the bending stiffness.