Assessing the effectiveness of newer treatments for rare diseases can be challenging because of the small number of patients treated at individual centers. We enrolled patients undergoing percutaneous transluminal septal myocardial ablation (PTSMA) for hypertrophic obstructive cardiomyopathy (HOCM) at five international centers (1 Japan, 2 United Kingdom, and 2 United States). Our study group developed standard data definitions regarding clinical symptom severity, previous HOCM treatment, procedure status, and outcome, and entered patient data directly into a shared, web-based registry system. In the first 10 months of 1998, 51 patients were enrolled in our registry, with 47 ultimately receiving the PTSMA procedure. Although HOCM is consider a single disease, there were significant differences among centers in patient characteristics (age, gender, and family history of HOCM), symptom severity, diagnostic techniques (measurements taken after provocation), and treatment (amount of alcohol used, timing of injection, and number of branches attempted).