<b><i>Aims:</i></b> To compare effectiveness of a 1-year weight loss program in moderately and severely obese patients. <b><i>Methods:</i></b> The study sample included 311 obese patients participating in a weight loss program, which comprised a 12-week weight reduction phase (low-calorie formula diet) and a 40-week weight maintenance phase. Body weight and glucose and lipid values were determined at the beginning of the program as well as after the weight reduction and the weight maintenance phase. Participants were analyzed according to their BMI class at baseline (30-34.9 kg/m²; 35-39.9 kg/m²; 40-44.9 kg/m²; 45-49.9 kg/m²; ≥50 kg/m²). Furthermore, moderately obese patients (BMI < 40 kg/m²) were compared to severely obese participants (BMI ≥ 40 kg/m²). <b><i>Results:</i></b> Out of 311 participants, 217 individuals completed the program. Their mean baseline BMI was 41.8 ± 0.5 kg/m².<b> </b>Average weight loss was 17.9 ± 0.6%, resulting in a BMI of 34.3 ± 0.4 kg/m² after 1 year (p < 0.001). Overall weight loss was not significantly different in moderately and severely obese participants. Yet, severely obese participants achieved greater weight loss during the weight maintenance phase than moderately obese participants (-3.1 ± 0.7% vs. -1.2 ± 0.6%; p = 0.04). Improvements in lipid profiles and glucose metabolism were found throughout all BMI classes. <b><i>Conclusion:</i></b> 1-year weight loss intervention improves body weight as well as lipid and glucose metabolism not only in moderately, but also in severely obese individuals.