Serum human placental lactogen (hPL) levels were studied in 806 women in late pregnancy. The hPL levels were positively correlated with birth weight but were unrelated to maternal age, parity, socioeconomic status, or the sex of the newborn. The hPL levels peaked at 37 weeks' gestation and then declined moderately. An individual's hPL levels in late pregnancy are quite constant week to week. Patients with severe chronic hypertension have low hPL values; those carrying twins have high values.