A physical examination checklist of 25 signs of Down's syndrome was used to predict the presence of absence of 21-trisomy in 291 individuals examined for suspected Downs syndrome. Using only total numbers of signs present, 21-trisomy was unambiguously predicted in about half those examined. Discriminant analysis using the most informative 10 signs misclassified only 11 of 169 infants aged 2 years or less, and allowed non-overlapping classification into Down's and non-Down's of almost three-fourths of suspected individuals. The risk for Down's syndrome in the overlap area was 58%.