Analysis of a considerable body of new data, based upon flow-visualization experiments, reveals a simple criterion for the occurrence of vortex breakdown at a fixed location in a tube: ReB ∼ Ω-3R−1, where Ω is the circulation number, R the ratio of the radial to tangential velocities in the inflow region, and ReB the pipe Reynolds number at which vortex breakdown occurs. The constant of proportionality is found to be practically independent both of the pipe flare angle α and the type of breakdown observed (bubble, spiral, etc.) although the latter is shown to depend on ReB. Theoretical support for the experimental results is derived from the analysis of Benjamin (1962) combined with similarity arguments.