We consider a single-input-single-output nonlinear system which can be represented by an input-output model. The system, which can be transformed into the normal form, is required to be minimum phase. The model contains unknown bounded disturbances. We assume that the reference signal and its derivatives are bounded. A high-gain observer is used to estimate derivatives of the tracking error while rejecting the effect of the disturbances. We design a globally bounded output-feedback variable structure controller that ensures tracking of the reference signal in the presence of unknown time-varying disturbances and modeling errors. We give regional as well as semiglobal results. We do not require exponential stability of the zero dynamics nor global growth conditions.