In the present study we have analyzed the prevalence and characteristics of the most relevant clinical and immunologic features in 1,000 patients with SLE. Several differences in the expression of the disease have been observed in relation to the patients' age at onset, sex, and autoantibody serology. The childhood-onset patients more often had malar rashes (55% vs 39%) and nephropathy (28% vs 15%) as presenting manifestations. During the evolution of the disease, these patients had an increased prevalence only of malar rash (79% vs 56%) and a lower prevalence of rheumatoid factor (6% vs 19%). The older-onset patients (age 50 or older) less often showed malar rash (21% vs 42%), arthritis (52% vs 71%), and nephropathy (3% vs 17%) as the first symptom. During the evolution of their disease, these patients had a decreased prevalence of malar rash (33% vs 60%), photosensitivity (29% vs 47%), arthritis (73% vs 85%), nephropathy (22% vs 41%), thrombosis (4% vs 15%), and anti-La antibodies (6% vs 20%), but an increased prevalence of sicca syndrome (33% vs 15%). Males more often had serositis (28% vs 16%) as a first symptom, but they presented with a lower prevalence of arthritis (74% vs 85%) during the evolution of the disease. The presence of ANA, a high titer of anti-dsDNA, rheumatoid factor, anti-ENA, and antiphospholipid antibodies also distinguished additional homogeneous SLE subsets of clinical significance.