||Aim. This review evaluated the effects of activity monitor-based counseling on physical activity (PA) and generic and disease-specific health-related outcomes in adults with diabetes mellitus type II (DMII), chronic obstructive pulmonary disease (COPD), or chronic heart failure (CHF). Methods. Four electronic databases were searched for randomized controlled trials using activity monitor-based counseling versus control intervention or usual care in adults with DMII, COPD, or CHF. Pooled effect sizes were calculated using a random effects model. Results. Twenty-four articles were included: 21 DMII studies and 3 COPD studies. No CHF studies were identified. Pooled analysis showed that activity monitor-based counseling resulted in a significantly greater improvement in PA compared to control intervention or usual care in DMII. Furthermore, these interventions had a beneficial effect on hemoglobin A1c (HbA1c), systolic blood pressure, and body mass index (BMI) (P < 0.05), whereas no differences were found on diastolic blood pressure, and health-related quality of life. Meta-analysis of COPD studies was not possible due to lack of available data. Conclusion. Activity monitor-based counseling had a beneficial effect on PA, HbA1c, systolic blood pressure, and BMI in patients with DMII. Data in patients with COPD and CHF are limited or non-existing, respectively.