Some twenty-five million roentgenographic examinations are performed annually in the United States (1). Thus a substantial fraction of the total population is exposed each year to whatever genetic hazards are associated with these procedures. In order to assess these hazards, it is necessary to know the gonad dose received during each type of examination and the distribution of the examinations in the population. This report presents phantom measurements of the gonad dose received during routine roentgenography together with some statistics of the distribution of roentgenographic studies. Experimental The phantom used in these experiments consisted of unit-density Masonite blocks measuring 10.6 × 2.5 × 2.5 cm. Chemical analysis of the material and a computation of the average atomic number, showed it to be tissue equivalent. The phantom was adjusted in all of its dimensions to the average trunk size of adults and children of different age groups (2). For the measurements, Victoreen thimble chambers of 0.1 r, 1.0 r, and 25 r capacity were used, all of which had been calibrated by the National Bureau of Standards. Removal of a single block from the phantom permitted placement of the dosimeters in appropriate positions. Localization of the female gonads was accomplished with the aid of hysterosal-pingograms and pelvic films, in the mid-frontal pelvic plane 9.0 cm, below the surface, 8.5 cm. above the inferior margin of the pubic symphysis, and 4.5 cm. from the mid-line on each side. The male gonad measurements were taken at the caudal mid-point of the trunk phantom 2.5 cm. beneath the anterior surface. The irradiation technics were those employed in diagnostic radiology at the UCLA Medical Center. Both conventional and high-voltage technics were used. The x-ray factors used for all the measurements were as follows: Target-to-film distance (except for chest) was always 40 inches, this choice being set by the use of a 16-to-1 Bucky grid; for the chest, the distance was 72 inches. Close coning was used in all cases. The added filtration was 3 mm. At. The half-value layers of the various kilovoltages employed are listed in Table VI. A small part of the data was derived from measurements on male students submitting to routine chest photofluorography in the UCLA Student Health Department. Calibrated Keleket pocket dosimeters were placed in the gonad region of the subjects anteriorly and posteriorly. Statistics on the age distribution of roentgenographic examinations were obtained from records of a large private general hospital, a county general hospital, and a children's clinic, all in the Los Angeles area. Results Table I gives the gonad dose, central beam dose, and x-ray factors for a number of examinations in children. Because of the large changes in body size that occur in childhood, the measurements were divided into three age groups: 0–2 years, 2–7 years, and 7–11 years.