Providing appropriate medical assistance in catastrophes in not an easy problem to cope with. Some of the problems involved in the task are the diversity of catastrophes and medical emergencies, the insufficient number of experts usually available during a given catastrophe, the necessity of improving medical performance time, and the lack of coordination of the resoures that may be used. In this repect, the application of both communication and computer technologies may provide important and useful support in most of the different phases involved in the management of catastrophes. In this paper we describe "MESTRIMAN", an expert system designed to provide medical assistance in catastrophes. "MESTRIMAN" is a module of SIAC, a computer system designed to manage relevant information and data about a wide range of different catastrophes and capable of performing as the central coordinator of the available resources.