['To investigate whether menstrual cycle length correlates with success rates at IVF/intracytoplasmic sperm injection (ICSI) and could be used as a marker of ovarian reserve.', 'Prospective observational study.', 'Private infertility centre.', 'A total of 6271 IVF/ICSI treatment cycles.', 'Self-reported mean number of menstrual days during the last year was recorded before initiation of IVF/ICSI treatment.', 'Relations between menstrual cycle length and pregnancy and delivery rates.', 'Increasing age was associated with a subtle shortening of mean menstrual cycle length. Menstrual cycle length correlated linearly with pregnancy and delivery rates, even after age adjustment. The chance of delivery after IVF/ICSI was almost doubled for women with a menstrual cycle length >34 days compared with women with a menstrual cycle length <26 days. Menstrual cycle length was also significantly associated with ovarian response to FSH/hMG stimulation and embryo quality.'] Is menstrual cycle length an age-independent marker of female fertility : results from 6271 treatment cycles of in vitro fertilization?