We report on the remarkable decrease in the platelet counts during pregnancy in two women with essential thrombocythaemia following treatment with recombinant interferon-alpha (r-IFN-alpha). Prior to pregnancy, the first patient was treated for 10 months with r-IFN-alpha 3 x 10(6) units/d six times per week, and the platelet count ranged between 750 and 800 x 10(9)/l. Starting from the sixth week of gestation, the platelet count decreased to normal levels and remained so, resulting in a lower r-IFN-alpha requirement. Following successful delivery of a healthy newborn an abrupt rise of the platelet count to pre-gestation values was observed, necessitating increased r-IFN-alpha dosage before pregnancy. The second patient when she became pregnant had been treated with r-IFN-alpha 3 x 10(6) units/d six times per week for 10 weeks. Starting from the 24th week of gestation the platelet count decreased, and despite reduction in the dose of r-IFN-alpha reached normal values at the time of delivery. The exact mechanism for the platelet count normalization during pregnancy is unclear, and several possibilities are discussed.