1. One hundred and ninety-five patients undergoing low-dose prednisone or prednisolone therapy were investigated. Blood pressure, weight, serum urea, sodium and potassium were recorded before therapy and again after at least 1 year of therapy. 2. The rise in both mean systolic and mean diastolic blood pressure was paralleled by an increase in the prevalence of arbitrarily defined hypertension. 3. There was no relationship between change of blood pressure and either dose of corticosteroid or duration of therapy. Blood pressure before therapy was the main determinant of the change in blood pressure. 4. Mean serum sodium levels rose slightly but serum potassium levels did not change during the follow-up period. There was no significant weight gain. 5. These results indicate that treatment of asthma and rheumatoid arthritis with prednisolone or prednisolone in low dose does not cause hypertension or biochemical features suggestive of mineralocorticoid excess.