A new hierarchical measure of cognitive complexity based on the repertory grid is described. This measure is considered to be both conceptually and empirically an advance over previous measures. It emphasizes that the structure of a subject's construct system is important, rather than merely its degree of differentiation. In this respect, it has advantages over previous measures based on the repertory grid. Three experiments are described which test the validity and reliability of the new measure. These experiments show that the hierarchical measure is reliable and is unrelated to a measure of differentiation based on the repertory grid. The hierarchical measure is shown to be positively related to performance on Harvey's ‘This I Believe’ test, which has been used as a projective measure of cognitive complexity. The hierarchical measure is also shown to be unrelated to performance on a verbal reasoning task.