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Theophylline has been used frequently in the treatment of patients with bronchial asthma and chronic obstructive pulmonary disease. The appearance of oral sustained-release products facilitated the regular use for maintenance of more stable therapeutic concentrations. It is well known that theophylline has a wide variety of side effects, including symptoms of the gastrointestinal and central nervous systems. These symptoms occur relatively early after initiation of medication. On the other hand, there are some reports that indicate potentially immunosuppressive aspects of this drug. Thus, long-term observation for ruling out of such side effects should be needed. Because asthmatic patients have a lower incidence of malignancies and this is possibly due to higher NK cell activity among those patients, we evaluated the influence of theophylline on NK cell activity in vitro and in vivo. We found an inhibitory effect of theophylline on NK cell activity in vitro and found that oral administration of theophylline also inhibited NK cell activity around the therapeutic concentrations in healthy volunteers. This inhibition was stronger than that induced by direct addition at the same concentrations in vitro.