Objective: To validate a simple, clinically relevant, and inexpensive test of aerobic power - the 10-meter incremental shuttle walk test (SWT) - in 2 separate patient populations. Design: Two-sample validity study. Setting: Physiotherapy department of major hospital in the United Kingdom. Patients: Convenience samples of rheumatoid arthritis (RA) patients (n = 10) and cardiac patients (n = 10). Intervention: Subjects were attached to a portable respiratory gas analyzer to measure oxygen uptake. They walked around an oval 10-meter course, starting at 0.5m/s, with velocity gradually increased by .17m/s increments for as long as they could, for up to 12 minutes. Main Outcome Measures: A subject's maximal rate of oxygen uptake during exercise (Vo2max) established with linear extrapolation was regressed against the number of shuttles completed (distance walked). An earlier study (n = 28) showed high levels of reliability and validity with linear extrapolation. Results: No significant linear relationship was found between Vo2max and the number of shuttles completed (R2; RA subjects = 9.7%, cardiac subjects = .03%, p > .05). Conclusion: These results do not support use of the SWT as a representative measure of aerobic power. Despite this finding, the advantages of developing a clinically viable alterative to costly laboratory testing warrants further study of the SWT in patient groups.