PURPOSE: Development of an implantable bladder volume sensor that could reduce complications and improve the quality of life for neurogenic bladder patients is assignment task that falls in the field of urology. Nevertheless, there is lack of research on whether biomaterials are biocompatible to the urinary bladder or not. Polyethylene glycol (PEG), polydimethylsiloxane (PDMS) and parylene-C are well known biocompatible materials in other fields of medicine. Because PEG is biodegradable and PDMS has a relatively low affinity to substrate with less durability than parylene-C, we evaluated the biocompatibility of parylene-C to the urinary bladde,r comparing of it to PEG and PDMS. MATERIALS AND METHODS: Nine rabbits were classified into three groups. Coin shaped aluminum substrates were affixed onto the external wall of the urinary bladder in each rabbit. At this point, the three rabbits which had substrates coated with PEG were assigned to group 1, those with PDMS were assigned to group 2 and those with parylene-C were assigned to group 3. In each group, one rabbit was sacrificed at one week, another rabbit was sacrificed at two weeks and the other rabbit was sacrificed at four weeks. At each time microscopic evaluation was done. To detect macrophages, we used fluorescence microscopy and applied MAC 387 staining. RESULTS: At one week, macrophage accumulation was observed on the external surface of the urinary bladder adjacent to the device no matter which material was used as a coating, but it had almost disappeared by four weeks. In addition, the inflammatory reaction was limited at the external surface of the urinary bladder, and did not expand into the muscular layer. CONCLUSIONS: With respect to biocompatibility, there was no difference among the three biomaterials. With its characteristics of durability and easy affinity regardless of the type of substrate, parylene-C would make an excellent coating material for a bio-device implantable into the urinary bladder.