Phosphodiesterases (PDEs) are key enzymes involved in the regulation of intracellular cyclic nucleotide metabolism. The aim of the present study was to identify and to characterize the PDE isoenzymes present in the human detrusor smooth muscle. Human detrusor PDE isoenzymes were separated by Q-Sepharose anion exchange and calmodulin-agarose affinity chromatography and characterized upon their kinetic characteristics and their sensitivity to allosteric modulators and inhibitors. All five presently known PDE isoenzyme families were identified: one high-affinity, low-Km calcium/calmodulin-stimulated PDE I with a slight preference for cGMP over cAMP, one cGMP-stimulated PDE II, one cGMP-inhibited PDE III, one cAMP-specific PDE IV and one cGMP-specific PDE IV. All five known PDE isoenzyme families exist in human detrusor smooth musculature. The kinetic characteristics, together with functional in vitro studies, suggest that the PDE I may be of importance in the intracellular regulation of the human detrusor smooth muscle tone.