Second-trimester hemoglobin (Hb) concentration was measured in 1535 pregnancies. Mean Hb concentration was significantly increased in women who developed pregnancy-induced hypertension (PIH) in the last trimester before the 37th week (p less than 0.01). A similar trend was also observed in nulliparous women who gave birth to infants with a birthweight below the 10th percentile, especially in smokers. The highest Hb concentrations were found in women with both PIH and intra-uterine growth retardation (IUGR). These findings are in agreement with previous observations in smaller studies and indicate that a reduced plasma volume may already be present in the second trimester, before PIH and/or IUGR appear.