Summary The diameter of the uterine isthmus was measured in 188 patients under different physiological conditions. It was found that measurements were reproducible with a low mean coefficient of variation (3 per cent). In the non‐pregnant patient the highest measurements were recorded during the menstrual period and the lowest in the preceding week. The isthmic diameter increased during the first trimester of pregnancy when it was slightly greater in parous than in nulliparous patients. Even 9 to 12 weeks after delivery, the diameter of the isthmus had not fallen to the size recorded in parous patients more than four months postpartum.