The reliability and predictive value of salivary theophylline levels was investigated in 58 children who were treated with oral sustained-release theophylline preparations for bronchial asthma. Serum and saliva levels were assessed by fluorescence polarization immunoassay (FPIA), and Seralyzer (reagent strip reflectance photometer method). Forty-six paired serum and saliva samples from 28 patients (Group 1) were used to develop a linear regression formula. This formula was tested on 130 paired serum and saliva samples from 32 patients (Group 2). When saliva theophylline levels were assayed using FPIA, 95% of serum levels were predicted within 30% of the measured value. Using Seralyzer, 95% of serum levels were predicted within 45% of the measured value. For patients with three and more paired serum and saliva specimens, individual regression formulas were determined showing considerable variation. This study indicates that salivary theophylline levels provide for an estimation of serum levels if FPIA is used. The Seralyzer technique is only useful for serum theophylline measurements. Using a special device it is now easier to obtain saliva samples than by conventional methods.