Age is often a dominant factor for women wanting to conceive. The objective of this study was to examine the outcome of an in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) program in relation to a woman's age. Between January 1995 and June 1997 we stimulated 2511 cycles. The mean age of the women was 34 years, with 21.9% under 30, 45.9% between 30 and 35, 24.5% between 36 and 39, and 7.7% over 39 years. All patients aged > or = 40 years had day 3 serum follicle-stimulating hormone (FSH) concentrations < 20 IU/l. The stimulation regimen consisted of 150-450 IU of human menopausal gonadotropin (hMG) or FSH combined with either clomiphene citrate (CC) or gonadotropin-releasing hormone agonist (GnRHa) in a short or long protocol. Age had a significantly negative effect on the stimulation and fertilization failure rates. The clinical pregnancy rate per transfer and the embryo implantation rate declined significantly from 29.4% and 18.9% in women < 30 years to 19.8% and 14.3% in patients between 30 and 35 years, 17.1% and 9.0% between 36 and 39 years and to 12.8% and 7.4% in those aged > or = 40 years. The spontaneous abortion rate was 14.9%, 16.5%, 22.4% and 33.2%, respectively. The clinical pregnancy rate per transfer reflected only imperfectly the performance of the older women because the discharge rate during stimulation and spontaneous abortions reduced the 'take home baby' rate to about 7% per cycle in patients aged > or = 40 years. It is very important in fertility practice to recognize the major impact of advancing maternal age.