A high-pressure liquid chromatographic method for the routine measurement of serum theophylline that is accurate, specific, and sensitive is presented. It is technically simple, requires only 50 μl serum, and takes relatively little technician time. Values for theophylline in sera obtained by this method correlate well with those obtained by use of a gas-liquid chromatographic method (r=0.980) and with values obtained by the commonly used spectrophotometric method (r=0.939) when the latter is free of interference. The assay has been employed for two years in a routine clinical chemistry laboratory, with interassay coefficients of variation of 2.4-3.2%. Interferences have been identified at an incidence rate of 3.8% of all specimens, and interfering substances include cephazolin, chloramphenicol, sulfamethoxazole, and sulfisoxazole. A calculation technic that allows a reliable and clinically useful estimation of a range of concentrations of theophylline in the serum when these interferences are present is described. In contrast, the incidence of interferences with the spectrophotometric method is 4.9%, and in this method the interferences prevent the analysis of theophylline. Of 33 samples having interferences by use of the spectrophotometric method, the high-pressure liquid chromatographic technic yielded precise values for theophylline in 21 and reliable estimates of ranges of concentrations of theophylline in the remaining 12.