Considering the narrow therapeutic index of theophylline and the low range between the safe and toxic serum concentrations of this drug, the study of its pharmacokinetic properties is necessary. However, considering the time consuming and expensive in vivo tests, quantitative correlation between in vivo bioavailability and in vitro dissolution tests can be used routinely in quality control tests of these drug products to predict the in vivo pharmacokinetic parameters. For this reason healthy human volunteers were used for in vivo studies and serum samples were analyzed by a fluorescence polarization immunoassay analysis (FPIA) method. The results showed that an open one compartmental model could best describe the pharmacokinetic properties of orally administered theophylline and aminophylline tablets. Linear regression analysis by least-square method showed a good correlation between some in vivo and in vitro parameters obtained from dissolution studies by rotating basket and paddle methods. D(30)% (percentage of drug dissolved in vitro after 30 min) and F(0.5)% (drug absorbed in vivo after half an hour calculated by Wagner-Nelson equation) showed best correlation (r=0. 99036). C(max) (maximum serum concentration) of this drug also correlates well with t(25%) (time required to dissolve 25% of the drug). The calculated correlation coefficients could best predict the actual values of some pharmacokinetic parameters; AUC(0-->infinity), AUC(0-->1), F(0.5)% and C(max).