An accurate evaluation of the size of the genitalia and its function is mandatory in judging abnormal growth of external genitalia from birth to adolescence as well as adulthood. This study was carried out to establish the standard values of penile and testicular sizes for normal Korean boys. Nine hundred and fifty-four Korean boys from birth to 15 years of age were subjected. The genital measurements were performed on the length and circumference of the flaccid and erect penis, and the length, width and depth of testis. The length of the penis was measured on the dorsal surface from the pubopenile skin junction to the tip of the glans with a sliding rule, and the circumference with a measuring tape. A sliding caliper was used for measurement of testis. Each testicular volume was calculated using the formula: V = 0.71 X L X W X D, where V is volume; L, length; W, width and D, depth. The mean of the two testes was taken as individual's testicular volume. The results are as follows. 1. The means, standard deviations and 95 percent confidence intervals of flaccid and erect penile length and circumference and testicular volume according to age were obtained. Except a slight increase in the penile length in the first four to five years the sizes of penis and testis did not change appreciably until prepubescence. With the onset of pubescence, there was a rapid spurt in penile size and testicular volume. 2. Marked differences in the erect penile length and circumference and testicular volume were observed between the two groups of the same ages with or without pubic hair. 3. A diagram of longitudinal median sections of testes in actual size arranged by age was obtained. 4. The right testis was larger than the left in 65.9%, the right testis was smaller than the left in 27.3% and the right and left testes were equal in 6.8% of boys studied. 5. Correlation coefficients(r) between height and erect penile length and circumference as well as testicular volume revealed 0.76, 0.74, 0.56, respectively. Correlation coefficients with weight were 0.75, 0.73, 0.55, respectively.