Inclusion Criteria:

1. Confirmed diagnosis of cystic fibrosis (sweat test/genotype)
2. Aged 8 years or older
3. Have FEV1 between 40% and 80% of predicted for height, age and gender OR a decrease in FEV1 of 20% or more than that recorded 6-12 months previously.

4. As determined by the investigator, are capable and willing to

   • Use the study diary as required for this protocol
   • Able to perform all of the techniques necessary to measure lung function
   • Able to administer the dry powder mannitol
5. Are capable of and have given informed consent
6. Clinically stable at study entry

Exclusion Criteria:

1. Investigators, site personnel directly affiliated with this study, and their immediate families.
2. Subjects under the age of 8 years.
3. Subjects with currently active asthma
4. Subjects using hypertonic saline treatment in the last 2 weeks
5. Considered "terminally ill" or listed for transplantation
6. Requiring home oxygen or assisted ventilation
7. Colonisation with Burkholderia cepacia
8. Significant episode of hemoptysis (>60 mls) in the previous 12 months
9. Myocardial Infarction in the six months prior to enrolment.
10. Cerebral Vascular Accident in the six months prior to enrolment.
11. Ocular surgery in the three months prior to enrolment.
12. Abdominal surgery in the three months prior to enrolment.
13. Subjects who are breast feeding or pregnant.
14. Female subjects of reproductive capability, not using a reliable form of contraception
15. Inability to obtain informed consent from the subject or subject's authorised representative.
16. Subjects who have participated in another investigative drug study parallel to, or within 4 weeks of study entry.
17. Known intolerance to mannitol or beta2 agonists.
18. Uncontrolled hypertension - systolic BP > 160 and or diastoli