To investigate the role of nutritional factors in growth and in the clinical, nurtitional, and respiratory status in cystic fibrosis, we studied 12 problem CF patients from six months before to six months after a period of supplemental parenteral nutrition. During the initial six months' observation period on appropriate conventional therapy, the patients (aged 0.5 to 11 years) had inadequate growth and weight gain, a total of 21 active pulmonary infections, and, despite dietary supplements, inadequate ad libitum nutrient intakes. After nutritional therapy, providing a balanced consistent hypercaloric intake for 21 days, catch-up weight gain occurred by one month and continued at six months; catch-up in linear growth was observed by three months and continued at six months. In addition, significantly fewer pulmonary infections were observed in the six months' post-therapy (n = 3), sustained and significant improvements were noted in clinical score and plumonary function, and there was a marked improvement in well-being and ad libitum nutrient intake. We conclude that adequate nutritional support can favorably affect growth, clinical status, and the course of chronic pulmonary disease in problem cases of CF.