By periodic intermenstrual pain we mean the recurring discomfort which some women feel half-way between their menstrual periods. The peculiar, persisting periodicity of this pain arouses the curiosity of intelligent patients, for, as one woman told us, it is "as regular as clock-work." Many women can foretell to the day the onset of the next menses from the date of this periodic intermenstrual pain. The Germans have called this syndrome mittelschmerz. The cause of this pain has never been determined. At the present time it is variously attributed to uterine contractions, to peristalsis of the tube as it propels the ovum toward the uterus and to ovulation. We are herewith presenting a study of sixty-one cases of this syndrome. After sketching the historical background of the problem, we shall outline the main features of normal ovulation and see whether periodic intermenstrual pain is in any way associated with ovulation. The