The prevalence of anaemia during pregnancy was investigated in relation to parasite and spleen rates of pregnant women living in a defined study area in rural Madang, Papua New Guinea. The effects in pregnancy of anaemia, iron deficiency and malaria on the foetus were investigated. There is a high prevalence of anaemia in this population, with 44% of primigravidae and 29% of multigravidae having severe anaemia [haemoglobin (Hb) less than 8 g dl-1] after 28 weeks gestation. The odds ratio for severe anaemia at 0-16 weeks gestation in pregnant compared to non-pregnant women was 4.7 (P less than 0.0001). Forty-seven per cent of primigravidae and 32% of multigravidae had evidence of iron deficiency with high free erythrocyte protoporphyrin values (greater than 35 micrograms dl-1 whole blood) at antenatal booking. The risk of severe anaemia was significantly associated with splenomegaly and iron deficiency for all gravidae (splenomegaly P less than 0.05; iron deficiency, P less than 0.0002). Hb values at delivery were higher than at first attendance, with the greatest difference between groups malaria-positive at booking and malaria-negative at delivery (primigravidae 1.5 g dl-1, P less than 0.01; multigravidae, 0.7 g dl-1, P less than 0.01), indicating that malaria prophylaxis was an important factor in controlling anaemia. Two Hb groups were defined on the basis of the cut-off at 8 g dl-1, which corresponded to the lower quartile value at booking and delivery. A significantly increased risk of low birthweight was shown for primigravidae with values below 8 g dl-1 (65% v. 27%, P less than 0.025), but the prematurity rate was not significantly increased, indicating that the majority of babies were growth-retarded. Early pregnancy anaemia and iron deficiency were related to the risk of low birthweight in primigravidae. Current parasitaemia at delivery appeared a less important factor, although primigravidae with severe anaemia and parasitaemia at delivery had the lowest birthweights. The extent to which malaria control, using drug treatment and chemoprophylaxis, can reduce the risk of low birthweight will vary in relation to the prevalence and causes of anaemia in women.