Our main aim was to evaluate whether maternal whole venous blood could be used for determination of fetal sex, when no enrichment of fetal cells was attempted and when "standard" interphase cytogenetics and PCR analysis were adopted. Altogether 39 pregnant women were studied by using ISH and 59 by using PCR. Out of the 59 pregnant women, 26 carried a male fetus and 33 a female fetus. By ISH, Y-positive cells were detected in 12 of 19 pregnancies with a male fetus and in two of the 20 pregnancies with a female fetus. The frequency of the fetal cells ranged from 1 in 639 to 1 in 100,000. By nested PCR with primers flanking a Y-specific repeat sequence, the positive band indicating a male fetus was found in one of the 26 pregnancies with a male fetus and in one of the 33 pregnancies with a female fetus. According to our results, fetal cells in maternal blood cannot be reliably used for prenatal diagnosis without enrichment of fetal cells.