We investigated the clinical significance of the close association of Sjögren's syndrome (SS) with mixed connective tissue disease (MCTD) by analyzing the clinical manifestations, sialographic findings and immunological parameters of MCTD and primary SS. The prevalence of sialectasia or SS in MCTD was significantly higher than in any other connective tissue diseases. The prevalence of Raynaud's phenomenon, swollen fingers, arthralgias, lymphadenopathy, sclerodactyly, muscle weakness, fever and erythema was significantly higher in MCTD than in primary SS. There were no significant differences between these manifestations in MCTD patients with sialectasia or SS, and those in MCTD patients without sialectasia or SS. Although the levels or prevalence of the erythrocyte sedimentation rate, CRP, antinuclear factor, anti-DNA antibody and anti-RNP antibody were significantly greater in MCTD than in primary SS, there were no significant differences in the levels or the prevalence of laboratory abnormalities between MCTD with sialectasia or SS, and MCTD without sialectasia or SS. Moreover, there was a strict dissociation between the occurrence of anti-RNP antibody and anti-SS-B antibody both in MCTD and primary SS. These results suggest that the association of secondary SS or sialectasia in MCTD, although more common than in other connective tissue diseases, is merely a consequence of MCTD and does not influence the clinical course of MCTD.