502 couples used the basal-body-temperature method of regulating births through 8294 cycles. The overall failure-rate in those confining coitus to the postovulatory phase of the menstrual cycle was 6·6 pregnancies per 100 women-years, while in those having coitus in the preovulatory and postovulatory phases it was 19·3 pregnancies per 100 women-years. The biological effectiveness of the method was greater than this, and was achieved by certain subgroups in the survey.