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Although high-intensity interval training (HIIT) and moderate-intensity continuous exercise have comparable health outcomes in the laboratory setting, effectiveness studies in real-world environments are lacking. The aim of this study was to determine the effectiveness of an unsupervised HIIT program in overweight/obese adults over 12 months.Two hundred and fifty overweight/obese adults could choose HIIT or current exercise guidelines of 30 min·d−1 moderate-intensity exercise. HIIT participants received a single training session and were advised to independently perform HIIT three times per week using a variety of protocols. Mixed models, with a random effect for participant, compared differences in weight, body composition, blood pressure, aerobic fitness, physical activity, and blood indices at 12 months, adjusting for relevant baseline variables.Forty-two percent (n = 104) of eligible participants chose HIIT in preference to current guidelines. At 12 months, there were no differences between exercise groups in weight (adjusted difference HIIT vs conventional = −0.44 kg; 95% confidence interval [CI] = −2.5 to 1.6) or visceral fat (−103 cm3; −256 to 49), although HIIT participants reported greater enjoyment of physical activity (P = 0.01). Evidence of adherence to ≥2 sessions per week of unsupervised HIIT (from HR monitoring) declined from 60.8% at baseline to 19.6% by 12 months. Participants remaining adherent to HIIT over 12 months (23%) were more likely to be male (67% vs 36%, P = 0.03), with greater reductions in weight (−2.7 kg; −5.2 to 0.2) and visceral fat (−292 cm3; −483 to −101) than nonadherent participants.HIIT was well accepted by overweight adults, and opting for HIIT as an alternative to standard exercise recommendations led to no difference in health outcomes after 12 months. Although regular participation in unsupervised HIIT declined rapidly, those apparently adherent to regular HIIT demonstrated beneficial weight loss and visceral fat reduction.Trial Registration: Australian New Zealand Clinical Trials Registry (ACTRN12615000010594), retrospectively registered.