Sixty-one of 173 patients with systemic lupus erythematosus followed for a mean of 13.9 years had severe infections which influenced their survival more than could be accounted for by the mortality (20 per cent) caused by the infections. Patients with infections had more SLE manifestations than patients without infections, and they died of lupus manifestations more often than patients without infections. Patients who went into a permanent remission and patients who died of lupus differed most markedly by the rates of infection. The rate of infection was increased more than tenfold in patients treated with high dosages of glucocorticoid compared with patients who received low dosages. Treatment with cytostatics influenced the rate of infections to a moderate degree. Nephropathy also influenced survival but half of the patients with nephropathy maintained a normal plasma creatinine in spite of the long observation period. 16 per cent of the patients with nephropathy died of kidney failure or are receiving chronic hemodialysis.