Four serum parameters were assayed weekly from the 4th to the 12th week of pregnancy and finally at 16 weeks, to assess their relative prognostic values for predicting pregnancy outcome. Of 85 pregnancies generated following treatment for infertility, 16 cases had blighted ova and subsequently aborted at a mean age of 9.9 ±0.5 weeks. Serum HCG concentrations differentiated (p < 0.005) between ongoing pregnancies and blighted ova as early as the 4th week which was often several weeks in advance of clinical abortion. PAPP-A, oestradiol-17/3 and progesterone did not differentiate between the 2 groups until 7 weeks (p < 0.005, p < 0.001 and p < 0.001 respectively). PAPP-A measurements detected ongoing pregnancies at week 4 (16.5 ±5 ug/l) but HCG remains the more sensitive diagnostic test. The lower limits of oestradiol-17/J and progesterone for ongoing pregnancies were 670 pmol/l and 37 nmol/1 respectively. The circulating concentrations of all 4 serum markers were unaffected by administration of me-droxyprogesterone acetate from 6 to 16 weeks in both ongoing and aborting pregnancies.