To determine the relation of growth and nutritional status to pulmonary function in young children with cystic fibrosis (CF). The relation of weight-for-age (WFA), height-for-age (HFA), percent ideal body weight (%IBW), and signs of lung disease at age 3 years with pulmonary function at age 6 years was assessed in 931 patients with CF. Associations of changes in WFA from age 3 to 6 on pulmonary function were also assessed. WFA, HFA, and %IBW were poorly associated with lung disease at age 3 years, but all were strongly associated with pulmonary function at age 6 years. Those with WFA below the 5th percentile at age 3 had lower pulmonary function at age 6 compared with those above the 75th percentile (FEV(1): 86 +/- 20 [SD] versus 102 +/- 18 % predicted, respectively). Pulmonary function was highest in those whose WFA remained >10th percentile from age 3 to 6 (FEV(1): 100 +/- 19 % predicted) and lowest in those who remained <10th percentile (84 +/- 21 % predicted). Patients with signs and symptoms of lung disease at age 3 years had lower pulmonary function at age 6 years. Aggressive intervention early in life aimed at growth and nutrition and/or lung disease may affect pulmonary function.