The aeroelastic FLEX 5 code and a semi-advanced rigid multibody model has been utilized for simulating drivetrain forces and moments in a real 500 kW wind turbine. Experimental validation is carried out with results based on known physical properties of the blades, tower, hub, gearbox, shaft and nacelle, etc. The multibody model consists of eight bodies, from rotor to generator, where most joints are made using simple constraints. Semi-advanced gear constraints are used for obtaining (worst-case) gear tooth reaction forces in the first stage of the planetary gearbox. This constraint is useful for not only transferring torque but also for calculating the gear tooth and internal body reaction forces. The method is appropriate for predicting gear tooth stresses without considering all the complexity of gear tooth geometries. This means that, e.g. gear tooth load-sharing and load-distribution among multiple planetary gears are not taken into account. Finite Element Method (FEM) calculations show that when the wind turbine runs close to the maximum wind speed, the maximum gear tooth stress is in the range of 500–700 MPa, which is considered to be realistic using a “worst-case” method. The presented method is based on a comprehensive description of the aerodynamic input, including inflow turbulence and shear, as well as various modifications for yaw, dynamic stall and dynamic inflow. Forces and torque from the aeroelastic and industry-accepted code FLEX 5 are used as input to the multibody program, where the gear constraint is formulated such that the maximum tooth forces are included directly in the solution. Copyright © 2013 John Wiley & Sons, Ltd.