The objective of this study was to develop a mathematical equation for the calculation of drug release from different shaped matrix tablets. By this way, release rate related to the geometric shape could be predicted with the help of the developed mathematical equation. So, drug release could be estimated before the dissolution. Hydroxypropylmethylcellulose (HPMC) E-50 as polymer and theophylline as active substance were used in the matrix tablets prepared for this purpose. Matrix tablets in three different geometrical shapes, namely in triangular, cylindrical and half-spherical forms were prepared by using two different drug-polymer ratio (1:0.5, 1:1) and diluent's in three different percentages (0, 20, 40%). Using rotating paddle and basket methods reported in USP XXIII carried out the release rate studies of these tablets. The Higuchi square-root time model best described the dissolution data. Differential scanning calorimetry (DSC) analysis was performed to identify any solid-state inactivation of the drug. The practical benefit of this work is to improve mathematical equation that can be used to predict accurately the required composition and in order to achieve the desired release profiles of different geometric shaped tablets. By using this equation new pharmaceutical products can be easily improved. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.