Rate nephelometric inhibition immunoassay (NIIA) was used to determine 94 serum theophylline concentrations, and these results were compared with those obtained using high performance liquid chromatography (HPLC) as a reference method. The measurements obtained by the nephelometric method were, on the average, 20% higher than those obtained by the chromatographic method (p less than 0.001). The coefficient of variation of the nephelometric method was 12.3%, compared with 3.9% for the chromatographic reference method. In the group of subjects having a serum concentration of caffeine greater than or equal to 0.5 microgram ml-1, the difference in serum theophylline concentration found between NIIA and HPLC correlated with serum caffeine concentration (r = 0.3755, df = 46, p less than 0.01). NIIA theophylline concentration was 8.8 +/- 3.2 micrograms ml-1 in serum from six additional patients receiving theophylline 9.1 +/- 3.4 micrograms ml-1 after adding 10 micrograms caffeine (NS), and 13.1 +/- 3.5 micrograms ml-1 after adding 10 micrograms 1,3-dimethyluric acid (p less than 0.001). We conclude that (a) the results obtained by the NIIA method are more variable and consistently higher than those obtained by the HPLC method and (b) 1,3-dimethyluric acid (a caffeine and theophylline metabolite) is responsible for this overevaluation.