In November 1981, a 77-yr-old woman consulted for myeloid metaplasia with fibrosis. A persistent hyperleucocytosis was treated with hydroxyurea from March, 1985 to March, 1989. At that time facial dyskinesia and polycythaemia developed. Investigations revealed a regression of myelofibrosis and a predominance of myeloid metaplasia in the liver. The mechanism of this event, rarely observed in myeloproliferative syndromes, is discussed in this new case.