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To investigate the development of the body mass index (BMI, weight/height2) in young adults, body height and weight of 518 men and 624 women, initially 19-31 years of age, were measured in spring 1980 and body weight was measured every six months from spring 1981 through spring 1984- The incidence of moderate overweight (BMI>25kg/m2) increased with age from 4.8 %/4yr to 15.5 %/4yr in men and was more stable, 5.3 %/4yr to 5.7 %/4yr, in women. The rate of gain in BMI was not found to differ according to the initial BMI in men, whereas in women a slight negative association existed between the initial BMI and its subsequent change. The within-subject standard deviation of the yearly measured BMI was 0.69kg/m2 in men and 0.74kg/m2 in women, and was larger both at higher levels of the BMI and at both extremes of the distribution of the rate of change in BMI. The considerable fluctuation in BMI may mask the slight increment in BMI and may thus hinder young adults from being aware of their becoming overweight.