Reproductive senescence has not been adequately explored as a possible end point for assessing toxicity to the reproductive system. We propose that urinary FSH be used as a biologic marker of ovarian function in women. A urinary marker would facilitate the incorporation of this measure into epidemiologic studies of reproductive toxicity. Although there is some evidence that FSH in serum is a good biologic marker of ovarian reserve, the relationship between serum FSH and urinary FSH has not been adequately explored. We found that urinary FSH and serum FSH were closely correlated in 50 consecutive women seen in clinical practice.