AIMS: To determine the prevalence of plasma cell monotypia in labial salivary gland tissue of patients with and without Sjögren's syndrome, and to evaluate its relation to the development of systemic monoclonal lymphoproliferative disorders. METHODS: A quantitative immunohistological study was performed on labial salivary gland tissue of 45 patients with Sjögren's syndrome, 18 with rheumatoid arthritis without Sjögren's syndrome, and 80 healthy controls. In none of the patients with Sjögren's syndrome was there evidence of systemic monoclonal lymphoproliferative disease at the time of biopsy. RESULTS: Monotypic plasma cell populations, defined by a kappa:lambda ratio of > or = 3, were only observed in older patients (above 43 years) with Sjögren's syndrome. In almost all these patients monotypic plasma cell populations were present in multiple labial salivary gland tissues and the IgM/kappa monotypia was observed most frequently. The prevalence of monotypic plasma cell populations in the group with Sjögren's syndrome was 22% (10/45) and there was no significant predilection for primary Sjögren's syndrome. Of special clinical interest was the observation that progression to systemic monoclonal lymphoproliferative disease had occurred exclusively in this subgroup of patients with Sjögren's syndrome, with a prevalence of 30% (3/10). CONCLUSION: Quantitative immunohistological examination of labial salivary gland tissues provides pathologists with a simple method to select those patients with Sjögren's syndrome who have an increased relative risk at the time of biopsy to develop benign or malignant lymphoproliferative disorders.