The present work was planned to research epidemiological and immunological features of systemic lupus erythematosus (SLE) in a Caucasian population from the north of Spain (Asturias). There is only one specialized immunology laboratory in this region where samples from all patients with a plausible or a firm diagnosis of SLE are referred for immunological analysis. Since 1992 we have reviewed registered data from samples submitted to the immunology laboratory with a firm, definitive diagnosis of SLE, based on the fulfillment of the American College of Rheumatology (ACR) criteria. We have constructed a database, which included 367 SLE patients. The point prevalence was 34.12/ 100 000 (95% CI: 30.63-37.61/100 000), whereas the incidence rate calculated during the last five years was 2.15/100 000/year (95% CI: 1.76-2.54/100 000/year). The female/male ratio varied according to the age at diagnosis, being maximum (50 : 1) between 22 and 28 years. The median age at diagnosis was significantly lower in females than in males. Immunological features also yielded sex and age peculiarities. The percentage of patients with anti-SSa antibodies yielded significant differences between males (18.6%) and females (34.6%). Anti-RNP and anti-Sm antibodies were more frequently present in childhood-onset patients, the difference with the oldest-onset group being statistically significant. Other analyses did not show significant differences, although, as a whole, we observed a trend towards a higher presence of autoantibodies related to an early disease onset.