A scoring system has been developed to grade the severity of acute systemic illness in babies under 6 months of age. Data were collected on 28 symptoms and 47 signs from 1007 babies with a spectrum of illness ranging from well to seriously ill. Ordinal regression analysis identified 19 symptoms and signs which in combination graded the severity of the illness most accurately. The coefficients were converted to scores. The higher the score the more serious the illness. When applied to a theoretical cohort of 10,000 babies at home, a score less than 8 has a specificity of 98%, and a score of 13 or more a sensitivity of 92%. The positive predictive value for serious illness increases from zero at a score of zero to approaching 100% at scores over 30. The scoring system has been developed into score cards for parents and professionals.