Critically ill patients admitted in I.C.U. often have multiple complicated problems which necessitate the use of a large number of drugs. The multiple potential interactions between substances and underlying pathologies as well as between substances themselves, obviate the need for a decision support system. We therefore developed a software package for medical treatment support in I.C.U. environment which is based on Microsoft Visual Basic 3.0 and it is organised around the commercially available RDBMS Access 2.0. The database consist of: a) all available substances, b) all generic names of medications available in Greece for each substance, c) incompatibilities (2,300 cases), d) interactions (50,000 cases) and e) cost information for each drug. This system assures the safety of the administered treatment, helps to adjust the dose according to the situation of each patient (anthropometric data, laboratory result, prognostic/severity score e.t.c.) and screens for possible interactions and incompatibilities between the administered drugs. It could also be used for education, treatment algorithms application and it will serve cost-reduction policy. It is a useful and powerful tool for ICU staff which does not impose additional work to the daily practice routine and it is currently under evaluation in clinical settings.