Summary o (1) A method of measuring the difference in height of the diaphragm as seen in consecutive x-ray films of the same person and using a standard radiological technique is described. (2) The range of variation in the height of the diaphragm has been measured in a control group of 30 non-pregnant women of childbearing age. (3) In a second group of 19 pregnant women it has been shown that there is a slight but significant rise in the level of the diaphragm in the later months of pregnancy. (4) In neither group was there a significant difference in the range of variation of movement between the left and right side of the diaphragm. (5) It is suggested that these structural alterations are the result of physiological changes of pregnancy similar to those responsible for the growth of the uterus and other muscles and the softening of pelvic ligaments and connective tissue. (6) The practical applications of these findings are discussed.