Open lung biopsy through a limited thoracotomy was used during the past three years in 11 selected infants and children. Biopsy was used not only for chronic undiagnosed lung diseases but also to accurately confirm suspected serious pulmonary disorders. Adequate lung tissue was obtained in all patients without significant morbidity and without mortality. The diagnoses in our eleven patients were Aspergillus pneumonitis, pulmonary blastomycosis, methotrexate pneumonitis, farmer's lung, visceral larva migrans, pulmonary allergic granulomatosis, Hamman-Rich interstitial fibrosis, idiopathic neonatal pulmonary fibrosis, interstitial pneumonia due to diffuse lupus erythematosis, and chronic bronchitis and bronchiolitis (two cases). In seven of these 11 patients, biopsy revealed a specific diagnosis. The technique permits early accurate diagnosis and treatment.