The relationship between haematologic parameters and pregnancy outcome was investigated in a prospective study, taking important covariates into consideration. A questionnaire asking information about smoking, alcohol consumption and iron intake was completed by a consecutive series of 796 ambulatory singleton pregnant women at 31-32 weeks of gestation. Subsequently, haematologic parameters were measured. After delivery, birth weight, gestation length and blood pressure values during pregnancy were abstracted from the obstetricians' records. Low birth weight (less than 2.500 g) and preterm birth (less than 37 weeks), were significantly more frequently seen in women with high haemoglobin (greater than or equal to 8.0 mmol/l) or high haematocrit (greater than 38%) values, with Mantel-Haenszel odds ratios varying from 2.4 to 4.2. Also a high erythrocyte count (greater than or equal to 4.5/pl) was correlated with low birth weight, while for mean corpuscular volume (MCV) no such relationships were found. Pregnancy induced hypertension was positively associated with haemoglobin, haematocrit and erythrocyte count, but could not fully explain the relationships between these parameters and adverse outcomes. Using multiple logistic regression with low birth weight and or preterm birth as the dependent variable, the effect of haematocrit still was demonstrable after controlling for smoking, hypertension, parity, alcohol consumption and iron intake. These results are in agreement with the hypothesis that a higher blood viscosity is a risk factor for suboptimal placenta-perfusion.