For early detection of iron deficiency during pregnancy ferritin, transferrin, serum iron, transferrin saturation and red blood cell (RBC) parameters were measured in 156 pregnant women. The geometric mean of ferritin-concentration was 11.7 +/- 2.7 ng/ml (mean +/- 1 s) which is about 46 ng/ml lower than that in non pregnant women. The duration of pregnancy, the number of pregnancies and the interval from the preceding pregnancy were all considered: 67% had iron deficiency, 18% prelatent, 27% latent and 22% manifest. On the basis of RBC analysis only 30% could be detected correctly. Ferritin detected 90% whereas serum iron, transferrin, transferrin saturation only detected 45% of the cases. The serum ferritin level did not correlate with the number of previous pregnancies but it did correlate with the interval to the last pregnancy. Serum ferritin declined sharply until the 28th week of pregnancy but only slightly thereafter. These data suggest at least one determination of serum ferritin concentration in pregnant women for a sufficient prophylaxis of iron deficiency during pregnancy. The latest time for the measurement should be between the 20th and 23rd week. An additional determination of serum ferritin during the early pregnancy is indicated if the time interval to the preceding pregnancy is shorter than three years.