Introduction: Adult obesity, among men and women, is a common phenomenon in the developed countries. A great amount of the obese women is trying to find the solution to the problem of obesity in the gym, exercising on the gym treadmill. High Intensity Interval Training (HIIT) seems to be extremely popular. Aim: To explore the results of the HIITon a gym treadmill in women aged 25-50 years, with specially designed protocols lasting 12 weeks. Material and Method: The following tools were necessary for the quantitative and the qualitative research: (1) Measurement Protocol, (2) Observation Protocol, (3) Axis Questionnaire used in semi- structured interviews. The measurement tools used were the skinfold caliper, the body weight scale and the height measurement scale. The clinical trials of the Experimental Group were performed on a gym treadmill, performing HIIT. Two Groups were formed with 15 women each: Experimental and Control. Both Groups followed a specially designed nutritional protocol. Results: HIIT resulted in a statistically significant reduction of the subcutaneous body fat and body weight. The reduction in body fat is differentiated by the classification of obesity. The observed reduction in body fat in the Experimental Women’s Group was statistically significant greater after the 3rd measurement. The number of the sessions seemed to be of great importance for the marginal means of the Body Mass Index (BMI). The «round apple” somatotype scored the largest reductions in the body fat in both Groups. Conclusions: HIIT on a gym treadmill was especially effective in reducing the body fat, the body weight, the abdominal skinfold, and the BMI and in defining the obesity. The best response on training on the gym treadmill, concerning the body fat reduction, was observed among the women with “round apple” somatotype. The Control Women’s Group with “round apple” somatotype, who had literally followed the specially designed nutritional protocol, resulted in the greatest body fat reduction, among the other somatotypes. Each next session seems to be statistically significant, whereas the final number of the sessions seems to be of great importance.