Silica exposure results in acute inflammatory response followed by chronic fibrotic change. The mechanism for the maintenance of silica-induced inflammation has not been understood yet. Apoptosis is a morphologically distinct form of programed cell death that plays major role during homeostasis and in many diseases including cancer, acquired immune deficiency syndrome and neurodegenerative disorders. Apoptosis is characterized by cell shrinkage, membrane blebbing and nuclear condensation. To demonstrate the involvement of apoptosis in underlying mechanism for the development of silica-induced pathological changes, this study was designed in vitro and in vivo models. In in vitro study, alveolar epithelial cell line (A549) was stimulated with silica and performed flow cytometry and DNA electrophoresis. In in vivo study, bronchoalveolar lavage (BAL) was done to count the total and apoptotic cells from silica-instilled rats. The results were as follows: 1. Apoptotic cell fraction of silica-treated groups (10 and 50 microgram/cm2) was significantly higher than that of control group. 2. Genomic DNA from silica-treated groups (10 and 50 microgram/cm2 ) showed DNA ladder in agarose gel electrophoresis, while group of 1 microgram/cm2 didn't. 3. Total cell number and apoptotic cell number of BAL fluid from silica-instilled rats (10, 20 and 40 mg/kg) were significantly higher than those of control. 4. Silica induced apoptosis of cells in BAL fluid was confirmed by microscopic observation with nuclear fragmentation. These results suggest that apoptosis may contribute to development of silica-induced pathological changes.