Assessment of the potential risk of in flight hypoxaemia in patients with cystic fibrosis is often based on a hypoxic challenge where individuals have saturations and/or blood gases taken before and after inspiring 15% normobaric oxygen. The aim of this study was to see if routine clinical measurements could predict the outcome of this test. This was a prospective study comparing the modified 6-min walking test, lung function, body mass index, Northern (N) and Shwachman-Kulczycki (SK) scores with a hypoxic challenge (flight test) in 69 adults attending the Leeds regional CF Unit. Although, there was a significant correlation between post flight test PaO(2) and N score (P=0.003), SK score (P=0.002), FVC % predicted (P=0.01), FEV(1) % predicted (P=0.002), resting saturations (P<0.001), 6 min saturation on walking test (P<0.001) and baseline PaO(2) (P<0.001), no single parameter could accurately predict all patients who desaturated during the flight test. No individual clinical parameter appears to fully predict the need for in flight oxygen but patients most at risk appear to have either a low FEV(1) (<60%), high N score (>13) or low baseline PaO(2) (<10.5 kPa).