In order to assess particular disorders of psychological development and functioning in children with developmental disorders, we have developed a new tool, the Behaviour Function Inventory (BFI), based on 11 neurophysiological functions, disorders of which are considered to contribute to the core autistic syndrome. This article reports the reliability and validity study of this new scale. Factorial analysis computed on the 55 initial items identified six main dimensions which we characterized and labelled: interaction dysfunction, praxis dysfunction, auditory dysfunction, attention dysfunction, islet of ability and emotional dysfunction. Relationships between these six new variables and diagnostic subgroups, and chronological and developmental age, are discussed. The BFI offers precise information of the functional symptomatology of autism, showing clear evidence of some disordered areas of functioning. This new approach may provide valuable information in clinical research, especially for psychopathology and physiopathology studies.