Evolution of fibro-osteosclerotic bone marrow lesions in the course of primary (idiopathic) osteomyelofibrosis (OMF) was studied in 36 patients (17 males, 19 females; median age 57 years) by morphometric evaluation of sequential trephine biopsies. The mean interval between first and terminal examination was 33 months (range 6 to 121 months). Two biopsies were performed in 31 and three and more in five patients. Morphometry consisted of a determination of argyrophilic (reticulin and collagen) fiber density, measured per area of hematopoiesis or marrow cellularity, and the calculation of the extent of trabecular bone tissue. In 13 of our 36 patients increase in reticulin and collagen deposits was only borderline to minimal during the observation period. On the other hand, in 23 of the 36 patients a slight to gross accumulation of reticulin and collagen fibers, partially associated with osteosclerotic changes was recognizable. No regression of fibrosis was encountered in our cohort of patients which included three cases with preceding low-dose busulfan therapy. Analysis of the different lengths of intervals between the first and the last biopsy and degree of fibrosis as well as osteosclerosis, suggested that alterations developed progressively, however, at an unpredictable and considerably varying rate. Thus our findings were not in keeping with several studies on smaller series of patients, which generally contested a progression of fibro-osteosclerotic lesions in OMF and additionally reported reversal of the pathology following chemotherapy.(ABSTRACT TRUNCATED AT 250 WORDS)