We evaluated 100 consecutive cases of lone aortic insufficiency (AI) for the prevalence of seronegative spondyloarthropathies. Four patients were found to have ankylosing spondylitis and three had Reiter's syndrome. Six of these seven patients had cardiac conduction abnormalities, four of which required permanent pacemaker insertion. All seven were found to have the HLA-B27 antigen, whereas of 89 patients tested with no evidence of spondylitis only five had the antigen. The seronegative spondyloarthropathies apparently are associated frequently with lone AI. The morbidity and mortality of these patients increases when they develop cardiovascular abnormalities including AI and heart block, which may dominate the clinical picture. The HLA-B27 antigen is not specifically associated with lone AI in the absence of spondylitis.