The occurrence of significant pulmonary hemorrhage associated with pulmonary arteriovenous malformations (PAVMs) and hereditary hemorrhagic telangiectasia (HHT) and the incidence of PAVMs in family members of patients with PAVMs and HHT are poorly defined. We reviewed our experience in 143 patients with PAVMs and HHT. Eleven (8 percent) of the 143 patients with HHT and PAVMs had a history of either massive hemoptysis or of hemothorax which required hospitalization. One patient died directly related to the pulmonary hemorrhage. There were four men and seven women. Three of the seven women experienced pulmonary hemorrhage during pregnancy. Seven of the 11 families participated in screening for PAVMs. Thirty-six (80 percent) of the 45 screened family members were found to have HHT. Thirteen (36 percent) of the 36 family members with HHT were proven to have PAVMs by pulmonary angiography. Pulmonary hemorrhage due to spontaneous rupture of the PAVM is a potentially life-threatening complication that should be treated aggressively with transcatheter embolotherapy. It occurs more frequently than previously recognized in patients with PAVMs and HHT. In addition, because of the increased incidence of PAVMs in family members of patients with HHT and PAVM, screening of family members with HHT is recommended especially in women of childbearing age.