The ideal body weight (kg) of each individual can be calculated by the following formula: ideal body mass index x the height (m)2, since body mass index is expressed by the body weight in kilogram divided by the height squared in meters. We investigated an ideal body mass index with respect to morbidity in 4565 Japanese men and women aged 30-59 years. Ten medical problems served as indices of morbidity: lung disease, heart disease, upper gastrointestinal disease, hypertension, renal disease, liver disease, hyperlipidemia, hyperuricemia, diabetes mellitus and anemia. The value of body mass index associated with the lowest morbidity was 22.2 kg/m2 in men and 21.9 kg/m2 in women, according to the quadratic regression curves relating body mass index to morbidity. From these findings, we propose that the ideal body weight is 22 x height (m)2. Our recommendations apply to the age group studied, namely 30-59 years.