Objective　To study the prognosis of polymyositis (PM) and dermatomyositis (DM) patients and its influence factors. 　Methods　Sixty DM and 30 PM patients diagnosed according to the diagnostic criteria of Bohan and Peter in our hospital during 2000-2008 were used as study subjects. The patients were followed up till their death or to August 2009. Gender

age of disease onset

disease course

serum creatine kinase

interstitial lung disease

heart involvement

connective tissue disease (CTD)

malignancies

and treatment with corticosteroids

immunoglobulin and immunosuppressive agents were assessed as predictive factors for the prognosis of patients. Characteristics of muscular biopsy from 20 cases were analyzed. 　Results　The median age of the 90 patients (29 males and 61 females) was 51 years (range 6-74 years). The male and female ratio was 1∶2. The most commonly involved muscles were the proximal muscles of limbs (83.3%)

followed by neck muscles (25.6%)

laryngea pharyngeal muscles (12.2%) and masticatory muscles (2.2%). Among the 42 patients（46.7%）with lung disease

interstitial lung disease and hear involvement were found in 29 (32.2%）and 13（14.4%）patients

respectively. Of the 13 patients complicated by connective tissue disease

DM and PM accompanying connective tissue disease were diagnosed in 9 and 4

respectively

and DM and PM accompanying malignancies were observed in 2 patients. Muscular or skin biopsy was performed for 23 patients

which showed typical inflammatory infiltration in 13

dermatomyositis in 3

and no significant lesion in 2 patients

respectively. Of the 18 patients who died during the follow-up

5 (16.67%) and 13 (21.67%) died of PM and DM

respectively. Seventy-two patients survived. Their 1-

5-

and 9- year survival rate was 90%

84.4%

and 80%

respectively. The complete and partial remission rate was 22.2% and 36.7%

respectively

with a relapse rate of 20%. Advanced age of disease onset (P=0.003 8)

interstitial lung disease (P=0.011 3) and malignancies (P=0.004 9) were main causes of death. Malignancy（RR=6.34，P=0.001 2）was the factor for poor prognosis and long-term treatment with hormones and immunosuppressive agents is the protective factor for PM and DM. 　Conclusion　Complete and partial remission can be achieved in 58.9% patients with DM and PM. Advanced age of disease onset

interstitial lung disease and malignancy are the factors for poor prognosis of such patients. Long-term treatment with corticosteroids and immunosuppressive agents are the protective factors.