Immunohistologic investigation of the sublabial salivary glands of 65 surgical patients who were free of systemic disease (control) and 19 patients with Sjögren's syndrome resulted in a new diagnostic criterion for SS that was more disease-specific than the widely used lymphocytic focus score criterion. Our bivariate classification criterion, which is based on the percentages of IgA- and IgG-containing plasma cells, has a specificity of 95.4% and a sensitivity of 100%, while its overall percentage of misclassification is 3.6%. This immunohistologic diagnostic criterion results in a reduction of the percentage of false-positive diagnoses from 9% to 4.6%.