The authors investigated the development of body weight of the population from six districts in the Czech Republic participating in the international WHO project MONICA. The examinations were made within the framework of epidemiological studies of risk factors of IHD in 1985 and 1988 in 1% independent population samples (range 25-64 years). The mean values of body mass index, BMI, increased in the above three-year period in men from 26.9 to 27.5 (p less than 0.001) in women from 26.9 to 27.4 (n. s.). The prevalence of obesity increased during the same period from 19.9% to 25.4% (p less than 0.001) in men and from 33.8% to 35.7% (n. s.) in women. In all decades of men and women there was a rising trend of body weight and increasing prevalence of obesity. With regard to the adverse effects of overweight on health and life span, the authors consider prevention of obesity as one of the foremost tasks of preventive medicine.