Objective To assess the significance of single fiber electromyography (SFEMG) in diagnosis of inflammatory myopathies and the correlation with other assistant examination findings.Methods SFEMG were recorded from the extensor digitorum communis of 34 patients with polymyositis or dermatomyositis and compared with the fmdings of routine electromyography (EMG), serum creatine kinase (CK) determination,and muscle biopsy.Results SFEMG recordings in 34 patients were all abnormal. The prominent feature was markedly increased fiber density (FD) with normally or mildly increased jitter. FD ranged from 1.0 to 6.0 (2.34±0.43). Jitter ranged from 5 to 78μs (41.6±10.3 μs). The potential pairs with jitter values greater than 55 μs ranged from 0% to 55% (7.7% ± 11.8%). Block was detected at one recording site in only one patient. Routine EMG demonstrated myogenic lesions in only 24 patients (70.6%). FD was a little higher in the normal-EMG subgroup or the neurogenic-EMG subgroup than myogenic-EMG subgroup but without statistical significance. Elevated CK levels were found in 75% patients (24/32). FD in the normal CK subgroup was significantly higher than that in the elevated CK subgroup (2.62±0.40 vs. 2.28±0.40, P ＜ 0.05). Muscle pathologies were consistent with the diagnosis of myositis in 75% (15/20).Conclusion SFEMG is of great value in the diagnosis and disease process understanding of inflammatory myopathies for the clinically suspected patients with normal routine EMG, CK levels, and muscle biopsies.