Although first reported 90 yr ago, the natural history of pulmonary arteriovenous malformations (PAVM) is still incompletely understood. We review historical perspectives and pathologic classification of this rare disease and identify etiologic factors and clinical features in the 450 reported cases. The many different diagnostic modalities currently available are described, and a logical sequence of investigations is suggested. Pulmonary angiography remains mandatory prior to definitive treatment. The available evidence strongly suggests that morbidity and mortality in untreated PAVM exceeds that associated with either surgical resection or embolic occlusion. Thus, definitive treatment should be offered in all cases, with the possible exception of asymptomatic patients with small lesions who have no evidence of Osler-Weber-Rendu disease. Embolotherapy is now the treatment of choice in institutions with the necessary expertise and facilities, notwithstanding the absence of long-term follow-up for patients treated this way. It is suggested that increased documentation of long-term morbidity and mortality in both treated and untreated patients with PAVM will lead to greater understanding and more effective management of this enigmatic condition.