In a prospective study, peripheral blood (PB) CD34 count, bone marrow histology and other clinical parameters were concurrently evaluated in 94 patients with myelofibrosis with myeloid metaplasia (MMM) and the study cohort followed for a minimum of 3 years. Median PB CD34 count was 0.0547 x 10(9)/l (range 0-5.345 x 10(9)/l) with 86% of the patients displaying above normal levels (>0.005 x 10(9)/l). In a multivariate analysis, only leucocyte count and PB blast percentage correlated with PB CD34 count in an independent and consistent fashion. After a median follow up of 41 months from PB CD34 analysis, 43 patients (46%) have died with a projected 5-year survival of 50%. In a univariate analysis, PB CD34 count above 0.1 x 10(9)/l correlated significantly with shortened survival, leukaemic transformation and clinical progression. However, such statistical significance was lost during multivariate analysis, which identified only anaemia and leucocytosis as independent risk factors for shortened survival. Furthermore, sequential analysis of PB CD34 count was performed in 53 patients and 43% of those who progressed clinically did not display a concomitant rise in their PB CD34 count. The current prospective study does not support an independent prognostic value for PB CD34 count in MMM.