Sixty-five patients with systemic lupus erythematosus were followed up for periods of 8 to 48 months. Sixty-six clinical exacerbations of the disease were observed and treated with various, non-randomized therapeutic regimens. The relationship between the results of immunological tests (DNA binding rate, serum levels of C3 and C4, presence of immune complexes and number of E rosettes) and the clinical and histological changes detected during treatment with corticosteroids or immunosuppressants was studied. In most cases a correlation was found between clinical and histological activities and the intensity of immunological reactions. Immunological abnormalities usually preceded clinical exacerbations but were rarely seen in patients with stable remission. However, exceptions occurred, which limit the value of these tests as sole therapeutic guide-lines in systemic lupus erythematosus.