Plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of tissue- and urokinase-type plasminogen activators, is considered a critical regulator of the fibrinolytic system. We previously reported a child with abnormal bleeding and complete PAI-1 deficiency caused by a frame-shift mutation in exon 4 of the PAI-1 gene. The purpose of this study was to provide genetic and clinical data on the extended pedigree of the original proband to better define the phenotype associated with PAI-1 deficiency. Allele-specific oligonucleotide hybridization was used to genotype individuals, and serum PAI-1 antigen was measured by enzyme-linked immunosorbent assay. By this approach we have identified 19 individuals who are heterozygous for the PAI-1 null allele and 7 homozygous individuals with complete PAI-1 deficiency. Clinical manifestations of PAI-1 deficiency were restricted to abnormal bleeding, which was observed only after trauma or surgery in homozygous affected individuals. A spectrum of bleeding patterns was observed, including intracranial and joint bleeding after mild trauma, delayed surgical bleeding, severe menstrual bleeding, and frequent bruising. Fibrinolysis inhibitors, including epsilon-aminocaproic acid and tranexamic acid, were effective in treating and preventing bleeding episodes. Other than abnormal bleeding, no significant developmental or other abnormalities were observed in homozygous PAI-1-deficient individuals. Heterozygous PAI-1 deficiency was not associated with abnormal bleeding, even after trauma or surgery. These observations define the clinical spectrum of PAI-1 deficiency and provide additional evidence to support the hypothesis that the primary function of plasminogen activator inhibitor-1 in vivo is to regulate vascular fibrinolysis.