Of four patients with Sjögren's syndrome, three had polymyositis and one had dermatomyositis. In all, deposition of IgG, IgA, IgM, and C3 was observed in muscle by immunofluorescent techniques. Serologic studies revealed elevated levels of serum IgG and IgM, rheumatoid factor, and antinuclear antibody with specificity for SS-A and SS-B antigens. In muscle there was a mononuclear cell infiltrate with plasma cell predominance around small vessels and capillaries. Ultrastructural changes in the vessels included reduplication of the basement membrane, endothelial thickening, and numerous tubuloreticular and dense inclusions. In two patients, electrondense deposits were noted in the microvasculature. This combination of immunoglobulin deposition in muscle, prominent microvascular changes, and characteristic serology suggests that the myositis in Sjögren's syndrome may result from small-vessel injury by autoantibodies or circulating immune complexes.