Several children presenting with mild symptoms of respiratory tract infection were diagnosed with unclassified interstitial pneumonia with fibrosis. Their clinical and radiological findings were similar to those of acute interstitial pneumonia, but there were some differences in the pathological findings. Unclassified interstitial pneumonia with fibrosis is characterized by histological findings of centrilobular distribution of alveolar damage and bronchiolar destruction with bronchiolar obliteration. This report describes two different series of familial cases of unclassified interstitial pneumonia with fibrosis, which developed almost simultaneously in the spring. Some of the individual cases showed rapidly progressive respiratory failure of unknown cause, with comparable clinical courses and similar radiological and pathological features, including lung fibrosis. Each family member was affected almost simultaneously in the spring, different kinds of viruses were detected in two patients, and all members were negative for bacterial infection, environmental and occupational agents, drugs, and radiation. These findings implicate a viral infection and/or processes related to a viral infection, such as an exaggerated or altered immune response, or an unknown inhaled environmental agent in the pathogenesis of unclassified interstitial pneumonia with fibrosis.