The corticosteroid (prednisone) effect on serum immunoglobulins in 9 atopic asthmatic patients who required corticosteroids for the control of asthma was evaluated. Serum immunoglobulins were determined before corticosteroids were administered, an average of 15 days while on corticosteroids, and again an average of 22 days after corticosteroids were discontinued. While on corticosteroids (averaging 16.8 mg prednisone daily) for 15 days, mean serum IgG was significantly decreased (-22%, p less than or equal to 0.01), mean serum IgA tended to be decreased (-10%), and mean serum IgM was essentially unchanged. Serum IgE was initially significantly increased (p less than 0.01) when compared to levels of other serum immunoglobulins (IgG,A,M). An average of 22 days after corticosteroids were discontinued, mean serum IgG was still significantly decreased (p less than 0.05), and mean serum IgA again tended to be decreased. Serum IgM remained unchanged and mean serum IgE now was significantly decreased (p less than 0.01). Corticosteroids appear to have a significant effect on levels of some serum immunoglobulins.