Recognizing the prodrome of a first psychotic episode prospectively creates the opportunity of intervention, which could delay, ameliorate or even prevent onset. Valid criteria and a reliable methodology for identifying possible prodromes are needed. This paper describes an instrument, the Comprehensive Assessment of At-Risk Mental States (CAARMS), which has been designed for such a purpose. It has two functions: (i) to assess psychopathology thought to indicate imminent development of a first-episode psychotic disorder; and (ii) to determine if an individual meets criteria for being at ultra high risk (UHR) for onset of first psychotic disorder. This paper describes the pilot evaluation of the CAARMS.