The article presents a study analyzing changes in body mass and depression and anxiety levels among the patients of the Obesity Clinic at the Municipal Hospital in Olsztyn. The aim of this study was to analyze long-term weight loss maintenance, and to evaluate depression and anxiety levels in people with obesity who participated in a weight loss treatment program in the Obesity Clinic in Olsztyn. Forty-five patients participated in a two-week weight loss program. During the treatment, the patients received support from a dietitian, a psychologist, a physician and a physiotherapist. The participants learned to consume regular meals five times a day (1200 kcal per day) to develop new eating habits, and they performed regular physical activity. During cognitive-behavioral therapy, the participants learned techniques that enabled them to reinforce new eating habits and adhere to a healthy diet. The patients were educated by all specialists during the two-week program in the obesity clinic. The study presents the results of the weight loss program in terms of body mass reduction and treatment of anxiety and depression which are often experienced by people with obesity who are unable to lose weight. The BMI and depression and anxiety levels were measured four times: before the weight loss program, at the end of the two-week program, as well as three and six months after the program. The results indicate that the program was effective in promoting sustainable weight loss.