This is a study to find out whether Pulmozyme is effective for clearing mucus from the airways of children with cystic fibrosis less than 3 ½ years of age. Pulmozyme is given using a nebulizer and is now widely used in older children and adults with cystic fibrosis. In adults and older children, studies have shown that daily use of Pulmozyme improves lung function and decreases the number of lung infections requiring hospital treatment. Pulmozyme has been approved by the Food and Drug Administration for use in children over 5 years old and adults with cystic fibrosis. Pulmozyme has also been approved by the FDA for use in children with cystic fibrosis less than 5 years old based upon studies showing that it is safe in this age group and that it does get into the airway tubes as well in infants and toddlers as it does in older children and adults. Currently Pulmozyme is not widely used in children with cystic fibrosis younger than 5 years because no study has clearly shown that inhaling Pulmozyme daily improves lung function or improves clearance of mucus from the airway tubes in very young children. This study will measure whether Pulmozyme improves lung function and mucous clearance from the lungs in children with cystic fibrosis less than 3 ½ years of age.

This study will compare Pulmozyme to a placebo. During the study infants and young children with cystic fibrosis will be treated with Pulmozyme for 6 months and placebo for 6 months. The study medicines will be inhaled at home once a day from a nebulizer for a period of one year. Half of the children will be treated with Pulmozyme for the first 6 months of the study and half will receive the placebo. At the 6 month point the group receiving Pulmozyme will be changed to the placebo and the group receiving placebo will be changed to Pulmozyme. The order of the 6 month treatment periods is randomized. This study is blinded. The study doctor and his staff will not know who is receiving Pulmozyme or placebo at any time during the study.

Whether Pulmozyme works will be measured using infant lung function tests and by doing a special 3-D x-ray of the child's chest (a high resolution CT or HRCT) at the beginning of the study, at 6 months and at 12 month after starting study. The study will not change the regular clinical care.