Maternal serum a-fetoprotein (AFP), human chorionic gonadotrophin (hCG), and unconjugated oestriol (uEJ levels were examined in 1632 women who had ovulation induction and 327 who had in vitro fertilization. There was a highly statistically significant increase in hCG and reduction in uE, among those with ovulation induction. The median levels were respectively 1.09 and 0.92 multiples of the normal gestation-specific median (MOM) based on a total of 34 582 women. Ovulation induction appeared to have no material effect on the median AFP level but this masked a significant increase when treatment was with Clomiphene (1.05 MOM) and a significant decrease when Pergonal was used (0.93 MOM). There was a highly statistically significant reduction in uE, among women having in vitro fertilization with a median level of 0.92 MOM. Those fertilized with a donor egg (21) had significantly higher AFP and uE, levels than when their own egg was used. Our results were confounded by differences in gravidity, but formally allowing for this factor did not materially change the findings. None of the observed effects is great enough to warrant routine adjustment of marker levels to allow for them. Moreover, women with positive Down's syndrome screening results can be reassured that this is unlikely to be due to them having had assisted reproduction.