Pleural biopsies from 21 patients with pleural mesothelioma and different asbestos exposure were analyzed by means of analytical electron microscopy with the aim of investigating the presence, quantity, types and sizes of asbestos fibers in pleural tissue. The majority of fibers found consisted of ultrathin (< 0.3 micron) and short (< 5 microns) fibers regardless of asbestos types and subject exposure. Concentrations appeared to be poorly related to the estimated exposure level. Fiber dimensions appeared to be the most important characteristic which determined their translocation in the pleural region.