OBJECTIVE High occupational physical activity (OPA) increases cardiovascular risk and aerobic exercise has been recommended for reducing this risk. This paper investigates the effects of an aerobic exercise intervention on 24-hour ambulatory blood pressure (ABP) among cleaners with high OPA. METHODS Hundred and sixteen cleaners between 18 and 65 years were randomized. During the 4-month intervention period, the aerobic exercise group (AE) (n = 57) performed worksite aerobic exercise (2 × 30 minutes/week), while the reference group (REF) (n = 59) attended lectures. Between-group differences in 4-month ABP changes were evaluated by intention-to-treat analysis using a repeated-measure 2 × 2 multiadjusted mixed-models design. RESULTS Relative to REF, 24-hour ABP signifcantly increased in AE: systolic 3.6 mm Hg (95% confdence interval (CI) 1.6-5.7) and diastolic 2.3 mm Hg (95% CI 0.9-3.8). Cleaners with high aerobic workload exhibited particularly high 24-hour ABP increases: systolic 6.0 mm Hg (95% CI 2.4-9.6), and diastolic 3.8 mm Hg (95% CI 1.3-6.4). CONCLUSION Aerobic exercise increased 24-hour ABP among cleaners. This adverse effect raises questions about the safety and intended benefts of aerobic exercise, especially among workers with high OPA and a demanding aerobic workload.