Calcitriol [1,25(OH)₂D₃] is usually investigated in studies on the preventive effect of activated vitamin D against interstitial pneumonia. Although cholecalciferol (vitamin D₃) can be easily obtained in the diet and has a longer half-life than calcitriol, there have been few investigations of its effect on interstitial pneumonia. We used human pulmonary fibroblast cell lines (HPFCs) and a mouse model of bleomycin-induced pulmonary fibrosis to evaluate whether vitamin D₃ was activated in the lungs and had a preventive effect against interstitial pneumonia. Expression of the vitamin D receptor gene and genes for enzymes metabolizing vitamin D was evaluated in two HPFCs, and the suppressive effect of vitamin D₃ on induction of inflammatory cytokines was also assessed. Gene expression of the vitamin D receptor and vitamin D-metabolizing enzymes was observed in both human pulmonary fibroblast cell lines. Vitamin D₃ suppressed bleomycin-induced expression of inflammatory cytokines and fibrosis markers by the HPFCs. In mice, symptoms of bleomycin-induced pulmonary fibrosis were improved and expression of fibrosis markers/fibrosis inducers was decreased by a high vitamin D₃ diet. Vitamin D₃ is activated locally in lung tissues, suggesting that high dietary intake of vitamin D₃ may have a preventive effect against interstitial pneumonia.