In the treatment of obstructive lung diseases theophylline is one of the most important drugs. But its therapeutic range is narrow, therefore attempts are made for an optimal therapy (dosage adjusted to the theophylline concentration in the serum, treatment with microcrystalline theophylline or retard preparations). Pharmacokinetic studies are undertaken with two theophylline formulations commercially available and the commonly used aminophylline to achieve the optimal therapeutic benefit. Regarding the results Aminophylline retard belongs to the formulations with diminished bioavailability and is therefore not satisfactorily qualified for an optimal treatment. On the other hand the development of a formulation of microcrystalline theophylline like Bronkodyl promises advantages for the long-continued treatment of obstructive lung diseases because of the smaller differences of the individual pharmacokinetic values from the median values.