The infant of an asthmatic patient showed irritability associated with ingestion of breast milk only on days when the mother was taking theophylline. Pharmacokinetic studies in this woman and in four other subjects revealed that theophylline distributes well into breast milk. The average milk to serum concentration ratio of the drug was about 0.7 and milk concentrations paralleled the time-course of serum and saliva concentrations. On a relative body weight basis, a nursing infant would usually receive less than 10% of the mother's dose of theophylline which is normally an inconsequential amount.