To test the hypothesis that aging of human gametes within the genital tract increases the chance of abortion, we measured the probabilities of abortion after insemination on a given day of the menstrual cycle in relation to the day of the shift in the basal body temperature in 965 patients. Cases came from family-planning and sterility clinics where basal body temperature and coital records are kept routinely. The probability of abortion diminished significantly (P < 0.001) as the shift in temperature was approached and then increased to its highest point (24 per cent) three days later.