No relationship was found between total fibre count and the latent period of the mesothelioma. Fibre counts were also performed on lung samples from 18 smear-positive and 30 smear-negative routine necropsies. Cases with numerous coated fibres in the smear gave total fibre counts similar to those of mesothelioma cases without asbestosis. Routine cases with only one fibre in the smear usually yielded only occasional fibres on analysis. Coated and uncoated fibres were found in 13 out of 30 smear-negative cases, indicating a true incidence of 56% of exposure to asbestos in the whole necropsy series. When compared with the mesothelioma series the difference in incidence of fibres remains statistically significant.