We describe here a series of 88 consecutive patients with bone marrow fibrosis. Primary causes for the fibrosis were discovered in 26% of the cases shortly after the initial diagnosis. Pathology review of the remaining cases revealed an 8% incidence of "hairy cell leukemia" that had escaped detection originally. The remaining cases, characterized as having "unexplained bone marrow fibrosis" or "idiopathic myelofibrosis," are the subject of this study. The clinical and laboratory findings are compared to those reported in previous series of selected cases with similar features in which patients were diagnosed as having "agnogenic myeloid metaplasia," "myelosclerosis," or "myelofibrosis." A brief summary of the treatment modalities used, and the clinical course and outcome of these patients are also presented. There was a marked variability in the clinical severity of the disease and in the survival of these patients. A detailed statistical analysis of 40 variables at the time of initial diagnosis showed that the factors that best predicted a poor survival were unexplained fever, weight loss, night sweats, anemia and thrombocytopenia. On the other hand, the size of the spleen or of the liver, the degree of immaturity of the peripheral blood white cells, and the degree of fibrosis or cellularity in the bone marrow biopsy were of no detectable prognostic significance. These findings suggest that in patients with unexplained fibrosis of the bone marrow (the syndrome of idiopathic myelofibrosis) a poor prognosis is not a direct consequence of the marrow fibrosis or the associated extramedullary hematopoiesis, but rather is related to the presence and/or the severity of some unexplained primary marrow defect, which is also often associated with the nonspecific symptoms of a systemic illness.