The absolute T-cell numbers and function, and the T-suppressor-cell subset were tested in eleven children with extrinsic asthma prior to and 1 month after theophylline treatment. Low mean T-suppressor-cell number and function was found in the asthmatic children, which returned to normal levels and function after 1 month of theophylline treatment. A normal mean T-cell number was found in this group of children prior to, and 1 month after, theophylline treatment. An obvious correlation between the clinical severity of the asthma and the number of T suppressor cells was found. It is suggested that theophylline most probably activated the T suppressor cells in the asthmatic children.