PIP: Fecundity has been reported to decline in women over 30 years of age. It is not known whether the decrease in fecundity is biologic or simply the consequence of diminished sexual activity. Artificial insemination with donor semen (AID) offers an opportunity to control certain variables in the study of female fecundity over time. 2193 nulliparous women who were receiving AID from 1973-80 at the Centres d'Etude et de Conservation du Sperme Humain (CECOS) and whose husbands were totally sterile were studied. The women were divided into 4 age groups: 25 years old or younger (n=371), 26-30 (n=1079), 31-35 (n=599), and 35 or older (n=144). At the end of the study period, the women were categorized into 4 groups, depending on the outcome: success (all pregnancies occurring during the study period), lost to follow-up (if the result of the last AID cycle was unknown), open case (result of last AID cycle was known but the next insemination procedure had not yet taken place), and dropout (discontinued treatment). The cumulative success rates were calculated ater 12 cycles with the life table technique adapted to AID as if there were no dropouts (theoretical cumulative rates). The Mantel-Haenzel test was used to compare the curves obtained from the cumulative rate as a function of the number of treatment cycles for the various age groups. The 4 curves differed significantly (chi-square=15.72, with 3 degrees of freedom; p0.01). The curves for the 2 age groups under 30 were very similar. Overall, the study shows that a decrease in fecundability (conception rate per cycle) as a function of a woman's age is slight but significant after 30 years of age and marked after 35 years. The probability of success of AID for 12 cycles declined to 61% (from 73% for those under 31 years old) for the 31-35 age group (p0.03) and to 54% (from 74% for those under 31 years old) for those over 35 (p0.001). 2 major problems encountered in studying variations in fecundity as a function of a woman's age are: 1) the need to separate the effect of the woman's age from associated variables such as coital pattern and husband's age, and 2) the woman's age itself, which could result in bias, since time introduces a type of selection. AID may provide the best means of minimizing the effects of associated variables and sources of bias.