From Jan 1978 to Dec. 1983, 39 cases of polymyositis and dermatomyositis which were diagnosed by clinical findings, electromyography, clinical laboratory and biopsy at Seoul National University Hospital were analysed in a retrospective study.

The results obtained were as follows:

1) The peak age incidence is the second decade(35.7%); female incidence is as much as 3 times that of males.

2) Among the 39 cases, 97.4% showed positive findings in symptoms, 92.2% in EMG examination, 86.4% in muscle enzyme and 74.3% in biopsy.

3) In EMG examination, abnormal findings appear in proximal muscle in a high ratio: Deltoid muscle showed abnormality in 88.2% of all examined muscles, Tensor Fascia Lata in 65.7% and Tibialis Anterior in 62.7%.

4) All cases in which the duration of symptoms was between 2 months and years showed abnormal EMG findings. But cases with less than 2 months or more than 2 years duration showed abnormal EMG findings in 80% of all patients. But it seems that the findings cannot be considered significant because of the small number of cases.