Mortality rates of 22,995 Finnish men aged 25 and over followed up for a median of 12 years were analyzed in relation to body mass index (BMI) at the initial examination. All-cause mortality followed a "U"-shaped distribution, being greatest for the thinnest and fattest men at all ages, or about 1.5-fold for those with BMI less than 19.0 kg/m2 and BMI greater than or equal to 34.0 kg/m2, as compared with men of normal weight (BMI 22.0-24.9 kg/m2). Mortality from cardiovascular diseases (CVD) increased with increasing BMI beyond the normal range. This depended mostly on the association of BMI with the biological risk factors of CVD. Mortality rates from CVD were also elevated among thin men under age 55, which could not be explained by the effect of the biological variables. Mortality rates from non-cardiovascular diseases, including cancers were inversely related to BMI among men of all ages. The high overall mortality of thin men was partly but not entirely attributable to smoking, low social class and antecedent disease. We conclude that both thinness and overweight are detrimental to longevity, but through differing mechanisms and disease patterns.