Theoretical analysis of the dynamic behavior of cavitating vortices requires knowledge of the velocity distribution in the vortex core. For that reason an existing similarity model for slender axisymmetric non-cavitating vortex cores has been extended with the viscous boundary conditions for a cavitating vortex. Cavitating similarity solutions exist for a vortex of which both core diameter and circulation grow with the square root of the axial coordinate. Results for this model are presented for varying cavitation numbers and Reynolds numbers. It is shown that viscosity may have a significant influence on the velocity distribution around a cavitating vortex.