Moderate intensity continuous aerobic training (MICAT) and high-intensity interval training low-volume (HIIT-low volume) improve the alterations associated to the metabolic syndrome (MS). Patients with MS have multiple morbidities and low fitness, which may predispose them to adverse events when exercising. The safety of these interventions on patients with MS has not been evaluated in depth. PURPOSE: to evaluate the safety of HIIT-low volume compared to MICAT in adults with MS. METHODS: a controlled, randomized, clinical trial using the minimization method. Sixty patients with MS, of both genders, 40-60 years old, were included. A clinical evaluation, biochemical tests, and an ergospirometry were carried out, before and after a treadmill exercise program of 12 weeks, 3 sessions/week. Volunteers in the intervention group received HIIT-low volume (n=29) in 22 min sessions that included six intervals at a load of 90% of maximum oxygen consumption (VO2max) for 1 min followed by 2 min at 50% of VO2max. The control group received MICAT (n=31) at an intensity of 60% of VO2max for 36 min. A new approach to record and classify the adverse events according to possible causality based in Naranjo´s algorithm was developed. RESULTS: patients had a mean age of 50.8±6.0 years, body mass index (BMI) of 30.6±4.0 kg.m-2, body fat percentage of 38.7±7.0% and VO2max of 29.0±6.3 mL O2.kg-1.min-1; 70% were women. In total, 60 clinical events were recorded in HIIT-low volume group and 48 in MICAT. Most of them (59.3%) were classified as general disease; there were no serious adverse reactions. Only 21 events (19.5%) were classified as an adverse reaction possibly related to exercise. HIIT-low volume and MICAT, had a similar frequency of musculoskeletal events (IRR: 1.1; 95% CI 0.6―1.8; p value=0.791), but higher of cardiovascular events (IRR: 2.9; 95% CI 0.4―22.8; p=0.310), after adjusting for age, sex and BMI (HIIT-low volume: chest pain (n=1) and symptoms of venous insufficiency of the lower limbs (n=2); MICAT: chest pain (n=1)). CONCLUSION: HIIT-low volume and MICAT are safe, however, we recommend a muscle-conditioning program prior to both and to avoid HIIT-low volume in treadmill in patients with history of venous insufficiency of the lower limbs. Colciencias 111562638757. Interinstitucional 2016-13041. Doctoral scholarships 727-2015