Introduction Polymyositis and Dermatomyositis (PM/DM) are both rare rheumatic and autoimmune diseases manifesting muscle inflammation and progressive weakness, DM includes skin inflammation, and are unknown cause and difficult to treat. PM/DM is designated as one of the intractable diseases by Ministry of Health, Labour and Welfare of Japan. And the government provides those patients a financial aid for the treatment as a specific disease treatment research program. Simultaneously most of the patients are registered to a database, which includes basic information of patients' characteristics. Using this database, we can clarify in detail the characteristics of the PM/DM patients.

Methods In Japan, 6327 PM/DM patients were registered in 2007. They are about 43% of the patients receiving financial aid by the government. We analysed the age, sex, age at onset, activity in daily life and clinical features.

Results In the registered patients, the number of male is 1735 and the female is 4592, sex ratio is 0.38. Age distribution by sex shows the mode age is 65–69 and 55–59 for male and female respectively. The age at onset of the disease distributes to wide range from 0–4 to 85−, the frequency goes up from the age 20s to the peak at age 50s, followed by descending trend. The onset age of female is a little younger than that of male.

Conclusion The registered large number of rare disease (PM/DM) patients' information provided fundamental features of the disease.