In Informal Structured Domains not all concepts and their relations are formally defined, the most of the problems does not have algorithms to obtain solutions, and the domain specialists use large amounts of tacit knowledge to solve problems.These characteristics generate ambiguous, inappropriate and incomplete requirements that could increase the development time of a software system. In order to minimize these problems, we propose a Requirements Engineering strategy based on knowledge management. As a case study, the strategy was used to the development of a software system, which evaluates the cognitive impairment in patients with sclerosis multiple.