A total of 120 samples of serum were obtained from 8 patients with COPD and submitted to a comparative analysis of serum theophylline levels employing the TDX and the EMIT methods of determination. At the same time, two different slow-release preparations of theophylline were compared, their kinetics investigated, and their effects tested using body plethysmography. For this open, randomized cross-over preliminary investigation, numerous inclusion and exclusion criteria were applied.