The startup flow of thixotropic yield stress fluids in tubes is studied numerically. In the situation of interest, the structured fluid is displaced by another fluid, under the application of a constant entrance pressure. The fluid is assumed to obey a recently proposed model for thixotropic yield-stress fluids that relies on a structure parameter. The momentum conservation principle is employed in a simplified form that arises by assuming quasi-steady flow, i.e. the shear stress radial distribution is assumed to be linear as it is in steady flow. Inertial effects are also included in the analysis. A numerical procedure was developed to integrate the differential equations that arise, and the average velocity as well as the radial distribution of the structure parameter are obtained for different combinations of the rheological and flow parameters, allowing the determination of the necessary conditions for a successful startup. Among other features, the approach is capable of predicting the "avalanche effect [5]," i.e. situations of no flow for long time periods and then the sudden onset of motion. (C) 2012 Elsevier B.V. All rights reserved.