The thickness of the glomerular basement membrane may vary not only in glomerular disease, but also in normal persons according to age and sex. But there has been no data on the normal thickness of the basement membrane in Korea. This study was designed to determine the glomerular basement membrane thickness as a reference value according to age and sex, in 50 cases of minimal change disease obtained from patients aged 2~67 years. Measurement of glomerular basement membrane was made on electron micrograph using an image analyzer. The thickness of each case was estimated by the arithmetic and harmonic mean methods. The mean thickness of the glomerular basement membrane was 291.9 47.9 nm by harmonic mean method and 284.2 43.7 nm by arithmetic mean method. And the harmonic mean thickness of the glomerular basement membrane according to age was 249.1 32.5 nm (1~5 years), 256.6 45.3 nm (6~10 years), 279.2 57.9 nm (11~15 years), 303.2 43.8 nm (16~20 years), 335.3 37.5 nm (21~30 years), and 291.1 22.5 nm (over 30 years), respectively. There was a trend that the thickness of glomerular basement membranes increased with the age till 30 years of age. There was no significant sex-related difference. In conclusion, the mean glomerular basement membrane thickness is comparable to the data from western people and shows a trend of increasing thickness according to the age.