The influence of women's age on the results of in vitro fertilization (IVF) was analyzed in 1801 women undergoing the procedure. Advancing age was found to be related to significant reduced success rates from an average of 30.1% per transfer below the age of 36 years to 15.9% per transfer at 37 years or more (P less than 0.001). The decrease was related to a reduction in oocyte production (five at 25 years or less, four below the age of 40 years, three at 40 years or more, and two in the 43 to 47-year group) and probably due to reduced implantation. It is concluded that a woman's age must be considered an important prognostic factor when IVF is suggested.