SUMMARYIndependent mobility is one of the major challenges for elderly. Assistive robots must guarantee user's safety. Existing methods to quantitatively express user's safety are not applicable to mobility assistance robots due to physical attachment. This paper proposes a danger index to estimate user's safety during mobility assistance. The formulation and validation of the index is achieved through extensive subjective and objective experimentation. A motion control method is also introduced that uses the proposed danger index for monitoring user's safety and initiating proper safety actions in case of danger. As an example, experimental results with SpiderBot-II are reported in this paper.