Primary myelofibrosis is predominantly a disease of old age, poor prognosis and no curable treatment. Thrombocytosis was observed in only 12% of patients. To our knowledge, there is only one reported case of a young woman with primary myelofibrosis who had a term pregnancy [1]. We report on a 29-year-old woman with thrombocytosis and medical history of two miscarriages in the last 2 years, the iirst at 30 weeks of gestation and the second at 27 weeks. Multiple placental infarctions were observed. She was without symptoms but with moderate splenomegaly 4.5 cm below left costal margin). The platelet count was 651 x 10(9)/L, WBC 7.2 x 10(9)/L with normal differential formula, and haemoglobin level 12 g/dl. Bone marrow biopsy showed fibrotic phase of primary myelofibrosis, with hyperplasia of megacaryocytes, decreased numbers of erythroid and granulocytic cells, and increased amounts of reticulin fibres. Cyctogenetic examination of the bone marrow showed normal female caryotype. Increased numbers of progenitors CFU-Mk, CFU-GM and BFU-E were observed in peripheral blood, and decreased numbers in bone marrow cultures. As the patient wished to become pregnant, the treatment with interferon-a (Roferon A) was started at a dose of 3 MU s.c., three times per week. The platelet count rapidly decreased at a level of 260-370 x 10(9)/L. The pregnancy was diagnosed 5 months later. At the 24 week of pregnancy, platelet count raised to 690 x 10(9)/l and the dose of interferon-a was augmented, 3 MU every day, until delivery. Foetal growth and placental circulation were monitored by serial ultrasonography. At the end of 34 weeks of pregnancy, it was noted that placental flow became insufficient, and after foetal lung maturity was stimulated with dexamethasone, Cesarean section was performed. Male baby was born, weighting 2000 g, with respiratory distress syndrome. This complication was successfully treated, and the child is now one year old, with normal growth and development. The mother is still on therapy with interferon-a, 3 MU, three times a week, and the last blood count was as follows: haemoglobin 10.7 g/dl, WBC 6.1 x 10(9)/L and platelet comt 437 x 10(9)/L. In conclusion, according to the clinical results of interferon-d in thrombocytosis and experimental studies which showed the absence of placental transfer of interferon-d, this therapy could be recommended to women with primary myelofibrosis who wish to have a baby.