The results of this study suggest that the additional determination of serum ferritin in pregnancy is necessary for a more reliable classification of iron deficiency. In 300 pregnant women the hematological values (Hb, MCH and serum ferritin) were determined toward the end of gestation. In 36 per cent of the women both Hb values greater than 11 g/dl and MCH values less than 28 pg fail to give evidence of an existing iron deficiency, indicated by serum ferritin values below 20 micrograms/l. The determination of serum ferritin is of particular relevance as a significant correlation was ascertained between low serum ferritin levels and the incidence of preterm labor: 52.3% of the women with serum ferritin levels below 10 micrograms/l and only 9.5% of the women with serum ferritin levels above 20 micrograms/l went into preterm labor. A convincing explanation for this has not yet been found.