Despite evidence that implicates transforming growth factor-α (TGF-α) in the pathogenesis of acute lung injury, the contribution of TGF-α to the fibroproliferative response is unknown. To determine whether the development of pulmonary fibrosis depends on TGF-α, we induced lung injury with bleomycin in TGF-α null-mutation transgenic mice and wild-type mice. Lung hydroxyproline content was 1.3, 1.2, and 1.6 times greater in wild-genotype mice than in TGF-α–deficient animals at Days 10, 21, and 28, respectively, after a single intratracheal injection of bleomycin. At Days 7 and 10 after bleomycin treatment, lung total RNA content was 1.5 times greater in wild-genotype mice than in TGF-α–deficient animals. There was no significant difference between mice of the two genotypes in lung total DNA content or nuclear labeling indices after bleomycin administration. Wild-genotype mice had significantly higher lung fibrosis scores at Days 7 and 14 after bleomycin treatment than did TGF-α–deficient animals. There was no significant difference between TGF-α–deficient mice and wild-genotype mice in lung inflammation scores after bleomycin administration. To determine whether expression of other members of the epidermal growth factor (EGF) family is increased after bleomycin-induced injury, we measured lung EGF and heparin-binding– epidermal growth factor (HB–EGF) mRNA levels. Steady-state HB–EGF mRNA levels were 321% and 478% of control values in bleomycin-treated lungs at Days 7 and 10, respectively, but were not significantly different in TGF-α–deficient and in wild-genotype mice. EGF mRNA was not detected in normal or bleomycin-treated lungs of mice of either genotype. These results show that TGF-α contributes significantly to the pathogenesis of pulmonary fibrosis after bleomycin-induced injury, and that compensatory increases in other EGF family members do not occur in TGF-α–deficient mice.