With the increasing importance of evidence-based practice and demonstrable clinical effectiveness, the need for valid outcome measures for all physiotherapy interventions is greater than ever. The VO2max remains the definitive measurement of aerobic capacity. However, there are serious practical and ethical concerns involved with applying such a demanding test to patients. As a consequence field tests are often applied with aerobic capacity inferred from performance. The most widely accepted and researched of these is the 10-metre incremental shuttle walk test (SWT) (Singh et al, 1992, 1994). In the present study the validity of a symptom-limited SWT as a measure of aerobic and functional capacity was examined with concurrent respiratory gas analysis. A VO2max value (aerobic capacity) established with linear extrapolation and observed VO2peak (functional capacity) were regressed against the number of shuttles completed. An earlier study (MacSween, 2001) established acceptable levels of reliability and validity of linear extrapolation. No statistically significant linear relationship was found between aerobic capacity and number of shuttles completed (R2 9.7%). Despite this finding, the advantages of developing an alternative to costly laboratory testing warrants further study of the SWT in patient groups. A statistically significant linear relationship was found between functional capacity and the number of shuttles completed (R2 61.4%). On the basis of the present results the SWT is a valid field test of functional capacity but not of aerobic capacity.