The clinical course of 103 patients (50 males, 53 females; median age 59 years) with idiopathic myelofibrosis (IMF) seen at our hospital between 1967 and 1986 was analyzed retrospectively. Common symptoms and signs at the time of diagnosis were: myelofibrosis (96%), splenomegaly (84%), anemia (81%), osteosclerosis (45%), malaise (41%) and leukocytosis (41%). It was possible to follow the majority of patients without treatment or with transfusion therapy only for prolonged periods of time. The use of cytostatic drugs and radiotherapy was restricted as much as possible. Probably due to this treatment strategy the incidence of acute leukemia was low (5%). Major thromboembolic complications were seen in 19% of the patients. Median survival of the patients was 4.3 years. The prognostic influence of several disease parameters determined at the time of diagnosis was tested: age, sex, leukocytes, platelets, hemoglobin, reticulocytes, LDH, ANP-score, spleen size and percentage of peripheral blood blasts + promyelocytes had no significant influence on the length of survival. Osteosclerosis, a presumed sign of advanced disease, was not correlated with survival either.