Dermal patterns in a group of Down's syndrome patients, a normal control population and a group of parents of Down's syndrome patients were studied in an attempt to identify an Index Score to be used in differentiating controls from parents of Down's syndrome children. Using only three patterns (simian crease, palmar hypothenar pattern and Cummins' Index), a parents' Index Score was established which correctly diagnosed 80.83% of controls and 79.17% of parents. The predictive value of this index and its interest in genetic counselling are discussed.