The circulating levels of human chorionic gonadotrophin (HCG), pregnancy-associated plasma protein-A (PAPP-A), Schwangerschaft protein 1 (SP-1), oestradiol and progesterone were measured in 81 pregnant patients between 4 and 11 weeks gestation, following in-vitro fertilization and embryo transfer. The patients were divided as follows: singleton anembryonic pregnancies, n = 22; singleton pregnancies which spontaneously aborted following the demonstration of fetal heart activity, n = 7; and normal singleton pregnancies, n = 52. The levels of all substances measured were significantly reduced in women with anembryonic compared to those with singleton pregnancies which proceeded to term. The serum levels of SP-1, weeks 6-8 (P < 0.01); HCG, weeks 6-8 (P < 0.05); oestradiol, weeks 5-8 (P < 0.05) and progesterone, weeks 6-8 (P < 0.05), were lower in anembryonic pregnancies than in those of pregnancies which spontaneously aborted. These differences may be a reflection of the fact that miscarriage, after the demonstration of fetal heart activity, represents fetal demise at a later stage in pregnancy. In anembryonic pregnancies, significant associations were found between HCG and both oestradiol and progesterone levels from weeks 6 and 8, suggesting that in the absence of an embryo, HCG is the prime determinant of steroid synthesis by the corpus luteum.