Abstract: A portable, battery powered microprocessor-based instrument for measuring basal body temperature on a daily basis and interpreting the measurement results in accordance with a cumulative sum (CUSUM) statistical test in order to recognize a shift in basal body temperature indicative of the beginning of a period of infertility. To minimize power consumption, CMOS logic devices are employed where possible, and the instrument is turned OFF except for a brief period of two or three minutes each day. The CUSUM test involves an ongoing calculation where each day's calculations depend upon results from previous days. The instrument includes an ultra low power CMOS RAM which is continuously powered and to which all necessary data is transferred under program control prior to the conclusion of each day's program execution. A predetermined range of temperature is established within which temperature readings must be in order to be accepted as valid.