It is demonstrated that weakly flocculated, concentrated colloidal dispersions show slip flow when sheared between smooth concentric cylinders. The precise pattern of behavior seen depends upon the stress and upon how long the flow is left to establish prior to measurement. With delay times of order hours, slip is not seen until a critical stress is exceeded (typically about 1 Pa) and, thus, the true low‐shear viscosity can be determined provided care is taken to ensure the stress does not exceed the critical level. With short delay times of order minutes, slip is seen irrespective of how small the stress is and the low‐shear viscosity can be underestimated by several orders of magnitude. Comparisons of flow curves obtained using smooth and roughened cylinders show that slip only occurs at the inner cylinder, and also that bulk flow is re‐established at higher stresses where the dispersions start to shear thin. The apparent low‐shear, relative viscositymeasured in the presence of slip appears, to a first approximation, to depend only upon the concentration of particles, and not on particle size, medium viscosity, or the strength of the attractive forces causing the flocculation. In consequence the slip coefficient appears to depend primarily on particle concentration. In contrast, the true low‐shear, relative viscosity (RLSV) is found to increase exponentially with the interaction strength. For example, an attractive well in the interparticle potential of order 10 kT gives rise to RLSV of order 106 in the concentrated, submicron dispersions studied here.