Earlier studies have shown that only 60% of sheep exposed to a given chrysotile exposure developed asbestosis. Analyses of lung lavage (BAL) fibre content early in the disease showed that, despite identical injected doses, the subset of sheep with interstitial lung disease had significantly more fibre retention. To determine if the fibre retention preceded or followed early disease, 15 were exposed at 10 day intervals to 100 mg chrysotile by intratracheal injection. Animals were studied at three month intervals by chest radiograph (CR) and BAL. At month 15, 10 sheep had definitely abnormal CR (group B) and five had normal CR (group A). Fibre analyses of BAL reproduced earlier finding of a higher level of fibre retention early in the disease, month 15: 92 +/- 2 f/microliter in group B v 35 +/- 19 in group A. Moreover, at month 3, when no disease was detectable, group B already had a significantly higher fibre retention level: 84 +/- 2 in group B v 52 +/- 3 in group A (p less than 0.05). These data clearly imply that high alveolar dust retention precedes the disease process and that alveolar dust clearance capacity may be a major determinant of asbestosis.