The objective of our study was to determine the clinical and immunological characteristics of primary Sjögren's syndrome (SS) in patients with a young onset of the disease. We included 144 consecutive patients (134 female and 10 male; mean age at onset 53 y; range 20-87 y) visited in our Units. All patients were white and fulfilled four or more of the diagnostic criteria for SS, proposed by the European Community Study Group in 1993. Disease onset was determined on the basis of the appearance of symptoms strongly suggestive of SS. In 13 (9%) patients, disease onset occurred before the age of 35. All were female and the disease onset occurred between 20-34 y (mean, 28 y). When compared with patients with older onset, patients with a young onset of the primary SS presented a higher prevalence of lymphadenopathy (54% vs 6%, P < 0.001), rheumatoid factor (70% vs 39%, P=0.034), anti-Ro/SS-A antibodies (70% vs 28%, P=0.004) and monoclonal immunoglobulins (23% vs 4%, P=0.02) in their sera. From the initial diagnosis of SS, three patients with a young-onset of the primary SS have developed lymphoproliferative disease at the time of the study, compared with one patient of the older-onset group (23% vs 1%, P=0.002). Our study shows several differences between younger and older onset patients, including a higher incidence of lymphomas in the younger, thus conferring to the age at onset of the disease a prognostic value.