Except rare instances of allogeneic stem cell transplantation, treatment of idiopathic myelofibrosis (IMF) is only palliative and based on cytostatic treatment (hydroxyurea and anagrelide), androgen therapy, steroids and splenectomy. Thalidomide is an anti-angiogenic and immunomodulatory drug with a wide spectrum of activities, which are not clearly understood. Current data suggest that the action of thalidomide is related to several different mechanisms, including suppression of tumor necrosis factor, effects on basic fibroblast growth factor, vascular endothelial growth factor, interleukins and interferons, downregulation of selected cell surface adhesion molecules, and changes in the lymphocyte subsets. We administered thalidomide to 16 patients with IMF (15 men, one women) who had transfusion-dependent anemia, thrombocytopenia or symptomatic splenomegaly. Median age was 59 yr (range: 52-78). Patients received thalidomide at an escalating dose from 100 to 400 mg/d (median 300 mg). The drug was discontinued in four patients because of progressive disease (two) or polyneuropathy (two). Other adverse effects were obstipation (10), fatigue (eight) and edema (two). Clinical response has now been observed for a median duration of 9 months (range: 3-20). Fifteen patients are evaluable. Anemia improved in six of 10 patients who were anemic. Platelet counts improved in five of seven patients with thrombocytopenia. Splenomegaly regressed in three of 13 patients. Lactate dehydrogenase (LDH) decreased in seven of 12 patients, but increased in four patients. LDH levels were not correlated with clinical response. In summary, thalidomide appears useful in the treatment of IMF.