The disturbance observer is a widely used form of a two degree of freedom control architecture which reduces sensitivity to modeling error while enhancing disturbance rejection properties. While standard robust stability analysis of the disturbance observer in the presence of unstructured modeling error is well known and results in a simple design guideline, similar results are lacking for the case of structured, real parametric uncertainty in the plant. Considering the case in which the plant has real parametric uncertainty, the real structured singular value method is applied to stability and performance robustness analysis, resulting in less conservative results. The model of an electrohydraulic positioning system with large parametric variation especially in its damping ratio is used to illustrate the concept. The results are extended to the case in which the plant under disturbance observer regulation is also under feedback control, as is usually the case in applications.