Obviously, the endocrine mechanisms involved in producing the normal, cyclic pattern of menstrual bleeding are exceedingly complex. A review of even our current, far from complete, knowledge of the regulation of follicular growth, cyclic selection of a single dominant follicle, ovulation, and the neuroendocrine control of all three mechanisms only serves to emphasize the myriad of endogenous and exogenous factors that may adversely affect such a delicate balance and be manifest in menstrual disturbance. Indeed, one may wonder that the menstrual cycle is cyclic and predictable at all. Nevertheless, the efficiency with which the system normally operates is striking. Its very complexity often makes disorders of menstrual function a not infrequent symptom of disease outside the reproductive tract, a fact that should stress the need for prompt and thorough evaluation.