Anti-histone antibodies have been detected in the sera of patients with various autoimmune diseases. The existence of anti-histone antibodies in patients with polymyositis/dermatomyositis, however, has not been reported. We found anti-histone antibodies in eight (17%) of 46 sera from patients with polymyositis/dermatomyositis by an enzyme-linked immunosorbent assay. One serum was positive for both IgG anti-histone antibodies and IgM anti-histone antibodies. Six sera were positive only for IgG anti-histone antibodies. One serum was positive only for IgM anti-histone antibodies. An indirect immunofluorescence analysis using HEp-2 cells as the substrate showed that all sera positive for anti-histone antibodies produced homogeneous nuclear fluorescence. This immunofluorescence pattern disappeared after absorption of anti-histone activity with total histones. An immunoblotting analysis demonstrated that the anti-histone antibodies were predominantly directed against histone H1 in all seven sera with IgG anti-histone antibodies. Weak reactivity with H2B and H4 were also found in three sera from the patients with polymyositis/dermatomyositis. Sera from two patients with polymyositis/dermatomyositis displayed anti-H2A and H3 activity. One of the two sera showed IgM anti-histone antibodies in the enzyme-linked immunosorbent assay reacted with H1, H2A, H2B, H3, and H4, whereas the other serum reacted with no fractions of total histones. The activity of anti-histone antibodies disappeared in immunoblotting after absorption with total histones. All of the patients with anti-histone antibodies were free from lung fibrosis or internal malignancies. Thus, our data indicate that the presence of anti-histone antibodies is classified as one of the serologic abnormalities observed in polymyositis/dermatomyositis.