A computer analysis was made of the data from a prospective study of the clinical course of systemic lupus erythematosus (SLE) in 234 patients followed for an average of 46 months. All fulfilled four ARA criteria for the diagnosis of SLE. Sixteen of the 234 patients were aged 51 or older. They were compared with the 218 younger SLE patients to determine the influence of age on the signs and symptoms of the disease, the organ systems involved, the laboratory data, amount and duration of corticosteroid or azathioprine therapy, and the prognosis. The older group showed more discoid lupus, photosensitivity and pulmonary fibrosis than did the younger group, but a similar incidence of malar rash, alopecia, arthritis, arthralgia, myalgia and serositis, and a lower incidence of oral ulcers, Raynaud's phenomenon, cutaneous vasculitis, neuropsychiatric manifestations, leukopenia, hypocomplementemia and profus proteinuria. The older patients needed a lower dosage of corticosteroids, and a shorter course of azathioprine therapy. These findings suggest a milder form of SLE with better response to therapy in the older group.