Regions of parametric resonance are predicted for the flexible blades of Horizontal Axis Wind Turbines subjected to periodic parametric loading due to gravity. The mathematical model consists of rigid blade flapping, rigid blade lagging and shaft torsion degrees of freedom. A hypothetical hinge offset, flapping and lagging stiffnesses, and aerodynamic loading are considered in the model. Also, hinge inclination which induces stiffness coupling is included in the analysis. The equations of motion (Mathieu/Hill's equations) are investigated using the Harmonic Balance Method for the determination of the main parametric resonance. The analysis has been carried out with the support of the computer algebra system REDUCE. Particular attention is given to: the importance of aerodynamic terms; the influence of coning angle; the influence of the flapping and lagging stiffnesses and the influence of the hinge inclination. The size and position of these regions of instability are significantly affected by these factors. Experimental results are presented from wind tunnel tests in which a Horizontal Axis Wind Turbine model of 1.82m diameter with flexible blades was investigated. The experimental results show clearly some regions of parametric resonance