Pregnancy in patients with primary thrombocythemia (PT) is reported to be often complicated by recurrent abortion and fetal growth retardation. Fifteen pregnancies in nine patients with PT are reported. Nine pregnancies had a good outcome, with the birth of a healthy infant. There were two spontaneous abortions and three intrauterine deaths. One pregnancy was electively terminated after extensive thrombosis in the splanchnic district requiring surgical entero-resection. In five pregnancies the mother received no treatment; in ten pregnancies acetylsalicylic acid (ASA) was prescribed to the mother as soon as she was found pregnant, subcutaneous heparin was added from the middle trimester in seven cases. In patients treated with ASA and subcutaneous heparin pregnancies had a good outcome. Administration of ASA and heparin during pregnancy appears to improve the outcome in patients with PT and can prevent severe maternal complications, but requires close monitoring.