The current best practice for assessing VO2max in adults with CF is a CPET using the Godfrey Protocol, a progressive and incremental maximal test performed on a cycle ergometer (13,14,15). Exercise testing should aim to achieve a maximal response within a time frame of 8-12 min and incremental protocols with stage duration of 1 minute are considered more efficient in eliciting the desired response within this time frame (14). During CPET, VO2max is determined while breath-by-breath gas analysis allows for a comprehensive assessment of exercise ventilation and circulation. This information can identify reasons for low exercise capacity and whether exercise limitation is due to deconditioning, or primarily within the respiratory, cardiovascular or peripheral systems. CPET is performed using a specialist ergometer, and requires specialist clinical expertise, monitoring and reporting equipment for interpretation of the test. The cost, space and expertise to carry out CPET in CF units around the world may limit its use for the regular assessment of exercise capacity in adults with CF (16).