OBJECTIVE: Healthy working-aged adults performed the modified Chester Step Test (mCST) to (1) determine the effect of repetition on test duration, (2) report cardiorespiratory and symptom responses, (3) establish a regression equation to estimate duration, and (4) calculate the minimal detectable change of the test. METHODS: In this observational study conducted in a hospital physical therapy, adult participants aged 25 to 65 years who were healthy performed the mCST twice. This submaximal test required participants to step on and off a 20-cm step at a standardized cadence that increased every 2 minutes. The criteria for test completion were either a heart rate equal to 80% of age-predicted maximum or the onset of intolerable symptoms. The primary measure was time to test completion during the mCST (seconds). Cardiorespiratory and symptom responses were also collected during the mCST. RESULTS: A total of 83 participants (40 men, mean [SD] age = 44  years) completed data collection. There was no systematic effect of test repetition with median test duration of the first test (522 seconds, range = 400-631 seconds) and second test (501 seconds, range = 403-631 seconds). The test elicited moderate symptoms of breathlessness and leg fatigue. In the multivariable model, age, sex, weight, and height were retained as significant predictors of test duration (R2 = 0.48). The minimal detectable change was 119 seconds. CONCLUSIONS: The mCST is a reliable and valid clinically applicable test of aerobic capacity in working-aged adults. Independent pretest predictors can be used to estimate the clinical time required to complete the test. IMPACT: The mCST was stable between test repetitions, suggesting no learning effect. For any given individual, a test duration change of 2 minutes represents change was beyond the natural variability. The mCST has good applicability to clinical settings.