We report four pregnancies in four patients with essential thrombocythemia (ET). In two cases pregnancy resulted in spontaneous abortion at 6 and 11 weeks of gestation, respectively. The other two had a full-term, normal delivery; one of them received no therapy during the pregnancy, whereas the other was treated with acetylsalicylic acid from 11 weeks for signs of threatened abortion. No thrombotic or hemorrhagic complications were observed in any of the patients during pregnancy or the post-partum period. We believe that pregnancy in patients with ET represents a risk. Close monitoring is therefore indispensable and antiaggregant therapy might be of benefit.