A crucial problem in psychiatry, affecting clinical work as well as research, is the generally low reliability of current psychiatric diagnostic procedures. This article describes the development and initial reliability studies of a set of specific diagnostic criteria for a selected group of functional psychiatric disorders, the Research Diagnostic Criteria (RDC). The RDC are being widely used to study a variety of research issues, particularly those related to genetics, psychobiology of selected mental disorders, and treatment outcome. The data presented here indicate high reliability for diagnostic judgments made using these criteria.