On the basis of observations in 117 children with cystic fibrosis, seen from January 1956 to June 1976, it is demonstrated that the relative underweight (weight loss corrected for height) is most pronounced in children with predominantly pulmonary sypmtoms. The degree of underweight closely correlates inversely with survival. Because of its prognostic value, it is recommended that this clinical parameter be included in the checkups which are periodically carried out on children suffering from cystic fibrosis.