The biological rhythm of females is closely related to the menstrual cycle, and this rhythm is believed to influence circadian changes in body temperature. This study investigated and compared the patterns of circadian changes in the body temperature of healthy adult females and patients suffering from premenstrual syndrome or major depression. Body temperature was measured both rectally and sublingually in healthy subjects, and only sublingually in the patients. During the luteal phase in healthy adult females, both the average and lowest nocturnal body temperatures increased, the amplitude of the circadian changes decreased, and the times of the lowest and highest temperatures within a 24-hour period were delayed by 2-3 h. In the patients, the amplitude decreased during disease periods, especially in the follicular phase, whereas in the luteal phase, circadian changes showed great variation each day, although the decrease in amplitude was not as remarkable. The results show that (i) the biological rhythm of females is intrinsically unstable in the luteal phase, although this rhythm is stable in the follicular phase; and, (ii) symptoms were often aggravated with the decreases in amplitude experienced in the luteal phase.