Serum placental protein 12 (PP12) levels were determined in 34 women with threatened miscarriage in whom a diagnosis of anembryonic pregnancy (n=19), missed abortion (n=4), complete or incomplete abortion (n=5), hydatidiform mole (n=1) and subsequent abortion of a live fetus (n=5), was made ultrasonically and confirmed by histological examination. Twenty-four of the 34 patients had one or more levels between the 10th and 90th centiles of the reference range. In 7 of the 19 patients (37%) with anembryonic pregnancy and in 3 of the 4 patients (75%) with a missed abortion, PP12 levels were markedly elevated. However, in cases where serial measurements were available (n=6), PP12 levels were seen to fall in all cases. It is concluded that pregnancies that have failed or will subsequently fail in the first or second trimester are not associated with depressed maternal PP12 levels, suggesting that the decidual synthesis of this protein is independent of the presence or viability of the fetus.